Basics 1st.
You mention stopping the car becoming a scrapper. Whats the body work like, possibly may need strengthening, will need some cutting and chopping probably round the gearbox area, but once you have fitted the engine and the box have you considered the extra BHP and Torque, beefier suspention and far superior brakes will be required. Can the diff cope with it. All this has to be considered 1st.
Brakes and suspenion can easily be sorted, tested and improved on a current car, more exspensive if you start with the engine.
Sourcing parts.
Rodley Motors
B&H Autocraft
Anglo Americam
DM Motors
Woolhouses
Coopers
PM me if you want more details
Most parts suppliers work from a massive catalouge for a lot of the aftermarket import stuff, it used to cost a fiver,
Rodlayes is probably the biggest importer for new stuff, woolhouses brings in 100's of engines a year.
Cheapest way is a donor car if you know what engine you want.
Try and get a chiltons engine book, it may well tell you every car the LS* engines were fitted in. Just had a look for the chiltons, seems they now have the Haines tittle in them - so ignore, the older ones used to cover a 7 year period at a time and were classed as car bibbles in thier day.