rebuild engine or ?
 

My 1989 300SE leaks oil around top & bottom seals, but otherwise runs well. Engine is very sound & I add oil as needed. With 120K miles on it I know there is still plenty of life in it. Issue is should I rebuild the engine or not? How much would it cost by a repair shop versus me doing it? I'm handy & have done one many years ago. Any advise would be helpful.

What top and bottom seals are you talking about?

You can pretty easily determine the life of the engine by doing a compression test. Usually a good measure of whether a rebuild would be worth it or not - but agreed with the post above. Which seals are leaking? Obviously some are much easier to change out than others.

How much oil are you losing?

If the motor is in such good shape, I would replace the leaking seals. Even if it is the front and the rear main seal.

Rebuilding a 120k mile motor would be foolish, just reseal it and motor on.

engine rebuild
 

It is the upper & lower gaskets that are leaking. I add a quart about every 1000 miles or so. Engine runs very smoothly. I need to replace the motor mounts too. Is there a resource available here that would serve as a step-by-step guide?

Who told you it was the upper and lower gaskets? You need someone knowledgeable in these cars to determine exactly what is leaking.

Resouces are here to walk you through it - just ask.

Sounds like a typical front cover leak which can be resealed very easily, with possibility of the valve guides being a little worn too which is normal for that mileage.

The bottom end of these engines are truly good for 300K+ easily. The top end needs to be done around 150K usually and you are good to go for the rest of the cars lifetime.

Should be able to get at least 200k out of the top end if oil changes were done

Correct. But most cars (like mine) had radiator failure of the plastic non-reinforced neck at ~120K which popped the headgasket's cherry. Necessitating top end work. There was a flaw in the original headgasket design, which is why one overheat on an original gasket spells r-e-b-u-i-l-d.

oil leaks
 

I have a 89 300se with almost 300,000 miles on it and all i have done is a valve job and reseal engine and beleive it or not it doesnt leak or smoke and bottom end is great,these engines last forever if properly maintained