Yeah it has to be said if you're starting from scratch on a 5.2!build yu really should be looking very hard at American/jap options instead.
ALTHOUGH that die hard lightweight beauty that is the rover v8 is a hard habit to shift.
Your p76 is something of a wildcard, easily the best all round option but like you have found out is a wallet shagger.
Basically theres no way for good power out of a rover v8 without around 2 grand minimum.
Any less than that and you're not making much power, and more and you could be said to be burning money for the sake of keeping the old rover.
I mean I must admit that I am dead sure for the weight of my vehicle now it simply needs more capacity that 3.5 litre, or at least more compression to make use of that capacity.
Bit of a rock and a hard place deciding what to do if we're honest, which brings is round to your original point of it taking ages to choose what to buy.
For me, despite all that we've said about large bore engines, I think top hat 4.6 is the best bang for buck, enough of them out there for cheap enough parts prices and good power available with decent reliability.
Still can't afford it though