The first on the agenda for me, would be bottom bracket and stem attention.
Whip off the cranks and get the BB out, see what you are dealing with. Even if it's blow out the curry powder, re grease and put back in.
If original or, a sympathetic swap out with a cartridge jobby. These old steel marin frames can fight you.
Stem too. Remove/clean/grease.
Upgrades....well apart from reinstate a DX rear mech, keep pretty much as, is.
Any high density foam grips will be OK.
Tyres, they may be marin ones, yet brittle and harsh to ride, consider a refresh.
Pedals. More of often than not I would buy vp du248 or similar. They look retro in design and work well.