whole bunch of ways you can go, but a Land Rover 200 tdi will provide:
-good power/weight ratio
-clean, modern, good cold starting diesel
-quieter, faster warm up (if heat matters) than older diesels
-a land rover engine in a land rover
-bolt up to a Series gearbox
-engine mounts are straightforward, easy mod.
-no trouble with axle or bonnet heights
-parts are easily available
-can get a Disco engine cheap in Uk or often here (I sold several last summer for $1400)
-can buy the twisted turbo exit from suppliers in UK to fit in Series if you have RHD
-or can go non turbi -200di and have an easy fit if RHD