You ask what is a good engine for your car and then you turn around and say you're getting tired of putting money into it. Kind of contradicting yourself there aren't ya
Anyhow,,, the main reason I replied to your question is to tell you yes your clutch and flywheel from your .15fe will transfer to a new engine BUT (and I emphasis that word) only if your new engine is equiped with a short crank shaft. If fact, you can probably only run the .15fe clutch and flywheel on you RS4.
As far as what engine, that's tough. If I were you I would check into the HPI Nitro Star engines or even their SS engines. Go to www.hpiracing.com
and look around for what they have. Also any Nova Rossi engine is good. Their website is www.novarossi.it/index_ing.html
. Just make sure the engine you buy has a short crank.