Hi Mate, and welcome!
Very difficult to answer question tbh...
Firstly, budget. An older x-flow car is more at the bottom end of potential westfield cars out there - don't be put off by that - I certainly wasnt! However, dependant on your size, if its a narrow, it may be a little tight size wise - I know mine is. However, payback is less weight and a nimble chassis, not to mention that glorious xflow noise. Bog standard engines so not a lot to check to (hopefully) make sure its a goodun....
More modern engines will see the budget rise, and possibly more to check on too...
If you have a reasonable knowledge of cars - mechanical noises and corrosion etc, then a westfield is pretty simple to have a good look around, and very easy / quick to jack up and have a good look underneath. Normal engine checks like state of oil, coolant checks, running noises and clutch biting points are all standard stuff - if your not too familiar with this, then take someone who is, or better still, someone from the forum local to the car will often volunteer to help.
Don't get too hung up on bhp, set up, chassis and gearing can make quite small bhp numbers seem a lot bigger on the road or track.
More modern engines on newer chassis are lovely bits of kit, but it all depends on what you want, what your budget is, and how much you want to tinker...
Generally, the cheaper the deal, the more it will cost to put right, upgrade etc, but there are great cars out there for not a lot.
value for money / smiles per mile, a westfield of any description takes some beating imho...
Good luck,
Mart.