1996 Chevy S-10 overheating

Engine Cooling problem
1996 Chevy S-10 6 cyl Automatic 210000 miles

Truck has been sitting for a year due to lack of funds and time to fix it. Last time I took it to the mechanic to fix my drive train (when I would go past 50mph it would shake alot). Not sure what he did exactly since I wasnt really into fixing my own stuff but time has passed and just starting to get a grip on how stuff works.

Now my problem. My truck overheats quick. I did the oil change and replaced filter. Since the truck was very very low on oil id figure maybe it was overheating due to old and very low oil. It helped just a bit but the gauge once again starting goin back up. It goes up rather quickly then comes back down and repeats.

my father says to replace the thermostat but I believe it was that was also one of its last repairs it had.

What else can I do? Flush the system and use rust removal fluids? Replace the radiator?

Also, While changing the oil, I saw old oil(sludge?) All over the bottom of the crankcase? Looked for leaks but I saw nothing. Ever since I did the oil change and cleaned up the mess down there it hasnt had any leaks? Coulda been a loose oil plug?

Thanks,
Frank