I am not a expert so you'll have to wait for the other members to give you there opinion but meanwhile here are a few things you'll need check when buying a Ke70. Parts for this car are easily available here and will most probably be cheap too since its a Toyota .
Cant really say about your budget cause it will depend on the car if you get lucky and find a nice example then you might not need to spend much on it, just look for the ride which has a good un-rusted body and no major accident, since you'll swap the engine I guess there will be no need to check the engine and transmission clutch etc but you might have to check the rear differential for defects and the shocks.