Hi,
I'm sorry to hear you had such a bad luck with the 300zx and 350z. Perhaps have a look on eBay, there are quite a few people breaking Zs and you might be able to find a used engine.
Maybe something like this, supply and fit, also 6 months warranty, perhaps you can negotiate it down a bit? https://www.ebay.co.uk/itm/303897676180
Alternatively see if @ZMANALEX or @P15UL T or Scott Soulby that break zeds all the time have any engines available?
We don't supply used engines, but if you're handy with the tools and want to fit the engine yourself, we would be happy to let you use our garage equipment, lift, tools etc over the weekend at no cost, that should save you a lot of money.