Wheel Arch Rust Assessment (Prior to purchase)

Well, it wouldn't put me off, but I like emerying down to bare metal etc.
 

It doesn't look bad to me, I think you tackle it by grinding back to clean metal.

Hello and welcome!
I think disking that down will go through in places. It wouldn'tbother me but if you aren't used to it then it may be tricky.
If that is all the rust showing anywhere it wouldn't put me off if the car is mechanically solid. Check for head gskt replacement during its life.
Keep us posted.

I’m not one for commenting on folks cars, we all have different standards of good & poor.
Those pictures, I would say, would be considered very encouraging & as the chaps say a bit of rubbing back & a paint should be relatively straightforward.
Do you know where to find the paint code?
M

You have to go back to bare metal, otherwise the rust will only return.

Btw.
Your approach on how to do it is spot on.
Only one of the arches looks like you could produce pin holes and if it does repair it from behind by cleaning it up and applying a small patch of glassfibre matting. Don't just put filler over the holes it will come through very quickly. You can underseal the matting.
This weather is not good for painting if you have to do it outside as there is too much moisture in the air and it will blush. Primer won't matter but paint and lacquer will.
Try get it inside and immediately before you spray warm the panel and you will be ok.
Any pics of the whole car?

Kidcreation, assuming you have not yet paid any money as deposit or outright purchase, I would make the following observations:
1. Whilst that rust could be removed, treated & painted, it looks very much like the surrounding sills and probably other areas have already been re-sprayed. If so, rust has re-appeared and that makes me doubt that the panels were properly prepared before that (cosmetic??) re-spray. If I were considering that car, I would insist on taking a good look underneath, to see what other rust is visible. The front mounts of both the front & rear sub-frames have a reputation for rotting first. There is also a notorious mud-trap at the back of the front wheel arches. Water & muck drop down the wheel-arch liner, and are trapped between plastic & steel. Rot follows.
2. How much service history, invoices or (ideally) a photo history of work being done. Cambelt changes and a properly replaced head gasket are the two "must haves" in my mind, unless the car is cheap.

Back in March 2021, I found rust in the same rear sill area as you. Photos of the rust spots and the re-spray are attached. At least your paint colour will be easier to match than my Starlight Silver!

It doesnt look too bad. MGFs / TFs are not unduly prone to serious rust in the bodywork unlike some of the cars from the same era so there is a very good chance that the arches there will be farly sound once that areas is stripped to bare metal. Are there any rust bubbles at the top of the arches or elsewhere on the car ?

As others have said do check when the cam belt was last changed and plan to change it f it was much more than 6 years ago.

How much is the seller asking ? What is the rest of the car like ?

Thanks TA22GT, and also thanks for your other comment approving of my proposed method i'm assuming these holes if small can be dealt with using your glassfibre suggestion, but at some point larger ones would require welding (and therefore professional assistance) assuming i ended up with any larger ones which of course i'd be praying against. Would i also be right in thinking it's just the lip where the bodywork folds around and not the actual "liner" surrounding the wheel arch itself? that presumably doesn't require any such treatments as its a different material? (yes i know i should have touched it while i was there andf then i'd know)
As for the rest of the car it's head gasket was done 5000 miles (2 years) ago by Trophy Cars as was the cam belt and water pump, its also had a new clutch, exhaust and stainless steel cooling pipes since then too and i didn't spot any additional rust. the underneath of the sills appeared to be black and felt very solid to the touch, i suspect the black is some sort of rust prevention treatment and it didn't appear to have bubbled (pardon my ignorance but could the black be some kind of treatment coating? and could this also be some of what can be seen where the arch meets the sill in the photo or is that just more rust?)

Thanks Airportable, off the top of my head i believe the paintcode is IAC, i do have it written down somewhere and some of these online paint places seem to at least claim they can do it so thats good. I'm hoping that because its on the inside i wouldn't need to worry about blending it in as much as you normally have to as i know that can be the tricky part and is where good painters really earn their living.
I think my biggest fear here was me not realising that this is actually very bad and could even be terminal, but if its not then i'm happy to take a go at trying to resolve it, you have to learn somewhere right?

Thanks EllisoJo, When i was looking at the car i couldn't see any signs of repaint but as i say i'm not that (read: "at all") experienced so i could be wrong, the underneath of the sills seem bubble free and strong, looked like they had possibly been treated in some way as they were black and i half assumed this "black" was also what was on some of the arches, especially where they meet the sill, though i suspect this was optimism talking.
In terms of history and invoices fortunately that's where the car shines! in the last 5000 miles (2 years) its had the cambelt and head gasket done by trophy cars, it's also had stainless steel cooling pipes which i beleive are a key upgrade for protection against this issue as well as new clutch from Rough Luck racing and a new exhaust

Thanks Notanumber, i didn't see any signs anywhere else on the car, that doesn't mean they weren't there of course and there was an advisory for front suspension arms corroded (but not weakened) so they'll need doing at some point.
The cambelt and head gasket were done 2 years ago with the car having covered 5000 miles since then, and in that time its also had stainless steel cooling pipes, a new exhaust and a new clutch.
The rest of the car seems pretty tidy from what i can tell, i don't have enough TF driving experience to know what's normal and what isn't to be honest and i assume most of the rattles fall into the category of normal, i did find the brake pedal didn't seem to have a huge amount of travel although looking back i'm wondering if i was simply not pressing it hard enough having been spoilt by modern cars and i didn't have any trouble stopping the car either way. definitely needs a new handbrake gaitor but other than that nothing really jumped out as bad.
The car has done just over 53000 miles and is currently up at £5000 though it appears the seller was trying for £6000 originally and from the way the seller was talking about wanting more easy to turn cars (fiestas etc) on his very tiny forecourt i suspect i could get a bit more off it too. It also includes the X-Power hoops and what i'm told is a rare X-Power tonneau cover designed to fit said hoops