In fairness £2500 should get a very nice example of any Ajax body car, other than a Sprint. Not so much rolling resto as "good car that need kept up" I'd think. One thing I would say is you'd probably get a nicer Dolly for your budget than a MGB GT which would probably also be lumbered with the naff "rubber" bumpers...
I paid £850 for my shambles of an 1850HL and that had a year's MOT, even if the underseal was structural, and it served me well for a couple of years. My 1300 was £1400 and has done over 10,000 miles in my ownership over the last three years despite being mechanically knackered and only needs a bit of bodywork now after several Scottish winters due to previous bodgery.
Mechanical work is easy on OHV cars, I've had the head off my 1300 on numerous occasions, replaced starters, ignition bits, brake system parts etc with no prior mechanical knowledge and just a Haynes manual, BL service manual and this forum (+ help from Dad)! Ancillaries on the OHC motor are just as easy but I'd not want to tear into the engine in all honesty and the concept of timing chain/water pump work scares me!
Engines will happily rock up 100,000 miles between rebuilds, if well looked after than can and will do more. OHV lumps tend to be more resilient to abuse although I hear 1500s don't much like high revs, my 1300 used to sit at 70mph just fine but 55mph felt much kinder!
Bodywork is a pain, all the panels are welded rather than bolted on so any replacement is going to be a cut and weld job. Sills, jacking points, subframe and chassis legs are really easy to check when viewing, beware super fresh underseal, might be hiding problems (ask how I know...)! Might be worth a check of the floorpan if you can lift the front carpets, the top of the A-pillar (open the door and check around the same level as the base of the windscreen) and the boot floor right at the back where the wheel arch is.
Other obvious bits are the front valance, wheel arches , bootlid and door bottoms although none of that is structural so I'd just stuff it full of fiberglass and filler like a cock.
Also to consider is where the car will live. I've kept my Dolomites outside and garaged and they deteriorate much faster when open to the elements (especially on the Scottish coast...), also working on the things at the side of the street at 11pm when it's freezing cold and raining is a miserable if character building experience which can't be avoided if it's your only form of transport. Also, OHV heaters are utterly crap, driving in snow is like driving a giant bar of soap, the whole car has two fuses that ruin your day if you don't keep spares, the quad round sealed beam headlights are rubbish, wipers only have two speeds (too fast and too slow), every other motorist will feel the need to pass you regardless of how fast you are going and the handbrake will need adjusting constantly
if you want it to do anything other than look nice in the cabin.
As a final word of warning, once you buy one you'll likely feel the compulsion to buy more and then they will ruin your life. Here are mine: