pygocentrus nattereri is a shoaling fish, actually a better way to put it would be pygocentrus nattereri (in fact this applies to all pygocentrus species) prefer swimming in groups. this makes them more secure, safety in numbers as there are many fish/non fish predators in the amazon pray on piranhas. and it's more efficient for them to hunt down food. can you keep just one? of course, but you won't know how that one fish will react being in your 36g. it could be one mean ass fish or could stay at the corning all day long. if you plan to keep just one fish, you should start with a serra. sanchezi. it will fit in your 36g for live.
as for getting a bigger tank and filter, you can always check craigslist or MFK's market place if you don't mind used ones. price of the tank setup depends on your location. my old 75g I paid $200, comes with tank, glass hood, 48" dual T5 fixture, DIY stand and one FX5 with medias. just have to look around, there are deals out there.