It honestly depends what you want to do with it.
I have an engine similar toy our spec, but i ported my own head, standard valves, single springs, 45mm weber, extractors, crow 51606 cam, 1300 pistons (40 thou over) and a bog standard bottom end (NO BALANCING).
I club day race mine and it hasn't let me down yet, will rev to 6,500 sometimes 7,000 with no problem.
I paid 300 for all machine work (bore, deck and chemical clean), then around 1,500 for parts then went in or on it.
At the end of the day it probably owes me 2,000, although its relatively stock, its great, nice powerband, and as far as i know reliable.
My advice would be, just don't get too carried away with other add ons, things that don't need doing. Just build it as an ultimate tight ass build, then go from there.
Cheers