the se-r would fit but you may need a new tranny. as for performance mods, if your gxe is a qg18de which i think it is then you just havnt been looking hard enough. you can get just as many parts for your qg18 than you can for a sr20. and ths way you dont need an expensive engine swap. if you can afford a swap then buy an older se-r. as for a turbo, there arent any bolt on kits buit if you cusomize your own its very duable.
at the top page of these forums is a mods list and a how to turbo your car. also if you go to this link then you can find what companies make parts specially for your b15.
Nissan Performance Magazine - Parts and Product Overview for the B15 Sentra
also try
b15sentra.net