you dont seem to really understand the mechanics of buying an old modified car.
they dont want to supply you a rwc. not because theyre dodgy, but because it is just not the way its done. you can buy the car, at a steal, for $3500. then, if you get the rwc yourself, probably have it on the road for another $600-$1000 including a few small repairs it might need for someone to be happy to give you a rwc.
but if they supply you a rwc with the car, then the car has to be 100% legit, and to be honest, with any car, this is a ridiculous amount of repairs, things like tears in seats, small specs of rust, every joint/bushes, absolutely no oil leaks ect ect. otherwise no rwc bloke will do it. selling a car with a rwc means, that if its not fully road worthy, the buyer has the right to chase up any further repairs needed to make it trully roadworthy with the seller/rwc guy.
ae86s that would pass this type of inspection are $10000+. so all of a sudden you cant actually afford and ae86 anymore, and you probably dont want one. as for that money, theyre not such a great deal. when you can get a boosted, coilied, lsd'd sr s13 for less.
on the other hand, if you buy the car, get it home, and then get a mobile rwc dude to come give you a safety cert, he can be lenient, as he knows that youre not going to come back to him later claiming the car isnt rw and he hasnt done his job properly. as he has done you a favor, to get your car on the road a bit cheaper, pretty much all he will want is to make sure your car is basically safe.