Whats your budget? You can spend $20k if you like, and get round 250rwkw
Many TD42's around with 160-200kw.
Your on a winner knowing its history, and "looking tired" does it need a $250-300 full interior and exterior detail?
Have you ever had the injectors and pump serviced? - CAN MAKE A HUGE DIFFERENCE! Should be done a few times in 300,000klms.
What tyres are you running? if they are 285's they will suck the power out of any engine, specially with a trailer.
How many tons does your patrol weigh? This is something people dont take into consideration.
Are you happy with how it goes without trailer?
Do you have $50k to spend on another 4wd? + $ to set it up again?
These engines regularly make 500,000klms with several on the forums making the 900,000 mark
To get your diff ratios sorted see Kent at Glenmount Automotive... Patrol owner and racer himself, great bloke with a hot secretary
If your car is already setup, and should be after all this time
and your happy with that, its DAMN expensive to start again - specially if you cannot take the goodies with you. I've done it and everything is "$1000ea".... :'( suspension, tyres, bar, winch etc
I ended up going with a Garrett Turbo update on mine, and I have from the Dyno on Dyno sheets:
265 AT's: 386 Ftlb (522.75Nm)
265 AT's: 130rwkw@102kph
285 MTRs: 312 Ftlb (422.54Nm)
285 MTRs: 111rwkw@108kph
It isnt cheap to do the conversion properly but hell worth it!
Not that its much, but with the loaded trailer, and loaded car on 285's heading away to Fraser or Flinders (cant remember), I still left a Terrano tryin to get round me as the lane closed out at the lights until I missed 3rd
You can pickup Factory GU Turbos on ebay complete ready to bolt on for ~$1000 and factory intercoolers for few hunge with piping and bracketery.
Sounds like you need to go test driving a turboed one and see if you are happy with it. If not, go see the bank manager