The oil leak is VERY likely the little U-shaped seal just under the small upper front timing cover. this is the number 1 spot for oil leaks on the M103 - replaced mine maybe 6-7 times in the last 26 years. The head gasket can leak oil from the pressurized oil passage near the front exhaust side; and it's hard to tell which is the culprit; but obviously do the $3 little seal first - do a search here for details, not a big job. Valve cover gasket runs down the rear when it oozes - piece of cake, replace on principle. My front crank seal has never leaked.
If the freeze plug you mentioned is the plug with a hose nipple for draining the coolant, then it's just screwed in, no gasket or o-ring. They can get really tight, but can be easily sealed with gasket goop.
Zerex G-05 coolant, still pretty easy to find. drain the rad by removing lower heater hose; drain block thru the above mentioned plug. water pump replacement requires a certain "agility".

Mine has ALWAYS had a warm starting glitch - let it sit more than 20 minutes and it will not hit on the first try, but always on the 2nd. I just hit it for about 1 second, turn it off and hit it again and it fires right up. In my humble opinion, this problem cannot be solved; the dealer worked it unsuccessfully about 25 years ago, and I've fiddled with it periodically ever since then, maybe 100 times; injectors, pump. filters, pressure, ignition, CIS, EHA; yadda yadda. Never gotten worse or better.

Oh, check the right motor mount, they collapse; should be able to put a finger between the heat shield and the lower metal section of the mount.
Cheap ones last two-six weeks!!!!! OEM are hard to find and cost $100.

Upper control arm bearings are the squeak monsters - Aftermarket ones last about a month, unless you take them apart and grease the heck out of them - then you can get a year or two.

Put the car in reverse and see how long it takes for the shift to take place. If it's longer than 2 seconds, prepare for tranny clutch plates - ouch!

Ah, old memories, but I've forgotten much more than I've learned in 26 years with my old '91. But it's all in here, lots of good search material.

Enjoy it; pretty good old cars.

DG