Nope for 500hp the twin 2860 -5's are fine. Stock manifolds and dumps on the R34 are fine too. R33 onwards also have the extended oil pump drive from factory. You can even use the factory oil and water lines. (basically everything bolts up nicely and looks stock)
You have to remember that to go a single turbo you have to buy new exhaust manifold, turbo, probably external gate (if you are going to the effort of going single why not), dump pipe, front pipe, intercooler piping, intake piping, oil feed + drain, and finally water lines.
Go the twins, its cheaper and gives just as much power as the 35R