83 300TD - My guess would be that, because 300,000 can be divided by 30,000, the service and adjustments should include everything listed in the maintenance booklet for that mileage. I'd also do the brake fluid and coolant replacements if they are due. If you are asking about other preventative items, it really depends upon how well it has been serviced in the past. I certainly wouldn't go replacing every hose, belt, glow plug, etc. unless they are tired or worn enough to justify it. Regarding the timing chain, you might check the stretch (actually, wear), but if it isn't making noise or worn too badly, I'd leave it alone too.

Mine is approaching 300,000 too.

Cheers,
Wes