I've had my 3.4 Colorado for coming up to 12 years now and 170k miles. It has had an LPG conversion since about 100k/6 years and has been fine so far. The 5VZ-FE engine is a pretty bombproof engine and all anecdotal evidence suggests that it copes well with LPG conversions without suffering from VSR. It's never let me down........so far.
I consistently get around 22mpg and 19mpg on long m'way runs on petrol and LPG respectively. It was about 1-2mpg better on the OEM 265/70R16 Dunflops before I fitted 265/75R16 ATs. However I'm happy to live with that penalty. Around town and on short journeys, expect that to suffer - 16/15mpg is about the best you can expect.
Not much to look for specifically on the 3.4V6 but at this age and if the mileage is high'ish - I would check the following;
Check for leaks around the rocker cover gaskets - mine started leaking (very slowly) around 150k, though never "used" any oil according to the dipstick.
Check the exhaust - only available from Mr Toyota and ££££s - so look at custom SS options, will be cheaper and better.
Check when the timing belt was changed - should be every 60k. Engine is non-interference, so not the end-of-the-world if it does let go. Probably good to change water pump every 2nd belt change. (At least that is what I did)
Make sure you extend the engine on the test run - should rev smoothly and comfortably past 4k rpm and on towards the red line (You may need to hold it in "2"nd gear to test this
) Also make sure that the gear changes are smooth too and the overdrive on/off is all working.
Changing the spark plugs can be a PITA but if you fit Iridium plugs, they're usually good for 30k+
As others have said, check the generic 90/95 (weak) points; Rad/autobox oil cooler leak, rear suspension turrets etc. The 3.4V6 (UK models) only came as automatics in VX spec, so make sure that all the toys work - heated seats, electric windows, mirrors, front seat adjustment, sunroof etc.
Depending on condition, you may need to budget for suspension renewal, brake overhaul and full service, some rust treatment etc. So take that into consideration and use that to negotiate if necessary.
Good Luck.
I'm sure I've forgotten something........