Looking at buying HJ75. 2H engine knock?

Good evening, everyone.

This afternoon I went and took a look at a local HJ75 ute. It doesn't seem to be in bad condition for it's age but it does have a bit over 600,000km on the odo. The guy had it running already when I arrived, apparently he had to jump start it as the battery had gone flat from sitting around parked for a while.

I noticed an intermittent engine knock while it was idling. Apparently the guy purchased it 4 months ago but wants to buy a 79 so he fits in it a bit better (I'm 6 foot and he had a few inches on me).

He has serviced it and given it a flush, which he claims has improved the knock situation. It hasn't been driven since the service so he thinks a long drive combined with another flush should bring it good. I'm not a mechanic but my better judgement is telling me to steer clear and ask some people more familiar with these motors.

Another thing I noticed was an occasional spark from the exhaust? I only noticed 2 or 3 within about 10 minutes. What do you guys think? He's offering it to me for 6 grand with a roadworthy. It has a brand new windscreen. Apparently the gearbox, suspension and shocks, injectors and fuel system (previous owner - no receipts). He's also letting me pay it off at $500 per week.

Any input on this intermittent knock thing which has me worried will be much appreciated.

Thanks.

Ben