My take, and by no means binding advice, lol
If I were constrained to one expenditure, it would be for either a new 1911 as you described, or I would buy a $1500 new hunting rifle that does what you want it to do.
I would NOT spend my allocated purchase on the upgrading an existing rifle. When it comes to rifles, $1500 will buy you a lot of rifle...a lot. I would much rather just spend the $1500 and get a new rifle. Personally I don't see the reason for customizing an old rifle like the 721 when you can have 2 guns, a nice old 721, and a nice new rifle from a semi-custom shop like Kimber or something similar, for the same money.
Just my $0.02