seriously dude we have had this convo before.
have u driven an m3, they might look nice but they are pretty boring (i take it u mean an e63 because if ur talking an e92 then i would choose that) they may look nice but they r not that good a car in my opinion (having had 3 beemers now) there gear box is a piece of shit as well unless u cruise it.
an r34 gtr is a unbelievable sporets car. now i do agree with u in terms of on paper the rb26 does look a lil small being only a 2.6 liter engine, however they r a work of art.
im doing the 3litre block and 26 head combo in my gtt atm and i think that is what u should do, or one of the many stroker kits available on the market. (tomei, hks, apexi, or the big boy os giken)
but
my first recomendation is
buy a gtr and see how u go from there, do some nice handling mods and get some good tyres on it and then tell us what u reckon. and some basic breathing mods as well to open it up a lil.
spool is currently working on a 3.4 litre stroker as well. but i dont know how far they have gone with it, i havnt seen any updates recently.