there are tons of factors that go into how much the bottom end can handle. everybody is looks for a magic HP number a certain part can handle and thats just a guideline
I can tell you the stock bottom end can survive on a 500 hp car. but maybe that car had a great tune and runs only in 60 degree weather and never sees traffic.
or how about a head and cam stock short block car with a big hit of nitrous that pushes it to 500 hp, but it hardly ever sees it.
or how about a poorly tuned hacked up supercharger install making 350 hp..
so my answer is the stock bottom end is good for 0-650 hp depending on more factors than you have hair on your body.
and my answer to the build the bottom end next question is NO.. I am sure you have other more pressing concerns than building up a N/A bottom end. the stock bottom end should be fine for you. then when you have the money to do a big bore setup then you do your bottom end.
quoted for truth. I dont see anyway that you could push to much HP N/A. I oly have a built bottom because I had spun a bearing due to overev and I wasnt gonna waste all that time pulling it down just to go back stock. and Ill prolly spray it later on.