Howdy from Oklahoma.
As for your question, the first things to come to mind are inspect cab corners, rocker panels and front floor panels. Additionally, some other common rust areas include corners of the doors, areas around the wheel arches, radiator core support and areas around/beneath the battery. Check the availability of rust repair panels, those panels are to service the most common rust prone areas on these trucks.
A common frame issue on these trucks are cracks in the front of the frame where the steering gear box attaches. If you have to convert to right hand drive, this may not be an issue.
Since you're tearing it all apart you will probably inspect/replace the body mounting bushings. Urethane bushings are a pretty common replacement for the factory rubber bushings.
Mechanical upgrades, especially the engine, is wholly subject to your desires. The powertrains on these trucks were/are fairly reliable and upgrade parts are generally inexpensive and plentiful.
Doing a search on this site for rust issues should give you some idea of the wide variety of issues that posters have dealt with.