first, welcome aboard.
since you already bought the boat, we cant talk you out of the purchase
being a 1983, its a stringer 800 drive. they were obsolete when the drive was new.
just in drive service, plan $1000. you most likely need ball gears, raw water pump with housing and you need to inspect the 3-piece vertical drive shaft. you will need to do the work yourself or find a grey-beard retired OMC technician that may do work on the side
you will need a factory manual. the only place to get a factory manual is here:
https://www.outboardbooks.com/
there are 3 gear boxes in the drive
does the tilt motor and clutch pack work? if not, you will need to replace the tilt motor and do not take the clutch pack apart without the required tools
inspect your boot
do not damage the lower shift cable. they are now $850
the trim pump and system is obsolete, however there are companies that you can get replacements.
if you have the bat-wing exhaust manifolds, plan on changing them
inspect the stringers and transom for rot. boats over 15 years of age have issues, yours is coming up to 38 years