I think "good" cars can still be found for around £15k. Much less than that and you will probably spend more than you save in getting it "good". Although shell condition is everything, the other bits can end up costing a fair bit too. If the car needs painting then that could add 4-6k to the cost. I dont think the usual mods (wheels, brakes, lowered slightly, front spoiler, webbers, exhaust, 280zx dist, ect.) make much difference to the price, but could add several thousand to the price if you want them done after. LHD solid shells and the ideal starting point (remember, not all LHD shells are solid) and even with the cost of converting to RHD, are probably the best way to end up with a "good" car. A 20k budget should sort this although you could buy one already done for this and just drive and enjoy. Pay around the 10k and you will probably spend at least that again in the next few years. Pay less than 10k and the costs escalate.