You can get a lot of cheap parts for these cars and engines, true, but you know most of them are utter crap. Quality comes at a price, i learned that the hard way. "buy cheap buy twice" has never been more true.
I also know the rebuild cost of these engines. Swapping bearings and hoping is not rebuilding. A crank is 700€ from nissan alone, forged pistons and rods are together are more than that. Gasket set is north of 150, oil pump is around 250, water pump and timing belt kit around 150, valves tend to need a change too. The list goes on, and that is even before paying a machinist to control and work on the block and head.
that being said, a forged CA18 is nearly unbreakable. Getting 450/500hp out of it is quite common without going into expensive fabwork. More than that and the budget skyrockets, but 700HP is still doable (without NOS, methanol, water spray nor race fuel). As long as there is oil, water, no det and breather system is clean, it won't break.
but there is no point in rebuilding it to stop at 300HP. Mine is still going strong after 220.000km of (ab)use. So either get a GWO engine (good luck with that) and do the usual stage 3 mods, or get one that needs a rebuild and go 400+. Although to be honest, i'd get an sr20 for anything north of 350hp if you want to stay jdm turbo 4 pots.
I am more and more inclined in building a cammed V8 now though
Now, seriously, the OP is looking at an engine with serious issues and should consider it as "rebuild needed". Frankly i would not touch it with a pole, sorting an engine can be a pain, and is surely the reason the seller says "it is a boost leak". Unless he is getting the car VERY cheap (cheap as in "a car without engine"), he should look for another one.