Firstly, the reason you buy a front cut is so you get a fully dressed motor with everything you need for a conversion.You will not save money trying to do it another way unless you buy an engine/gearbox combo from someone who is doing a complete engine conversion into their silvia or 180. After you buy all the little bits and pieces( if you can find them) That you didn't realise you needed , it wont end up any cheaper, just more of a headache. There is demand for the leftover bits and pieces of a front cut that you can sell to get some money back.
Secondly, if you are still on your L's, why not do yourself a favour and stick with an a15 with a bit of work to start with. An sr20 manual in a sunny, will , IMO, be dangerous for a person who hasn't much driving experience, and other road users, Much the same as a v8 commodore or falcon.
But thats just my 2 cents, take it or leave it...