Find out about barrel condition and headspacing. Both must be OK, at a minimum.
The era of the dirt cheap surplus Mauser is about gone.
Get one with a good barrel, cause most owners seem unwilling to rebarrel surplus rifles. They'd rather shoot corroded 80 year old ammo out of a sewer-pipe smoothbore, and then complain about the bad groups, instead.
Your particular source for Kar-98k's, I am unaware of their reputation. If you think you are getting an actual Kar-98k, just make sure it is an actual German one. I think the Yugoslavians have sent a bunch of their post-war communist mfgd. look-alike versions over in recent years. If you are fine with this provenance, then buy it and shoot it. If that isn't OK for whatever reason, then be pickier and email the sellers and quiz them.
Either road is fine by me. It's your money, so buy whichever one (in good, shootable condition) that makes you smile the biggest. The Juggy ones may be your best value. Those guys
loved 7.92x57
As a seat of the pants estimate, I think a decent Kar98K will set you back $300, nowadays.
Other Ideas:
Mosin Nagants are a good deal right now.
.303 Enfields with good barrels are 1st rate.
Swedish Mauser is an excellent small ring Mauser.
My particular Turkish Mauser is horrid. Buy a good one or save your money.
Springfields are valued sky high.
The DCM is still selling Garands for a song.
Buy the rifle of your dreams, and then take up reloading for it. I do.
There are scads of interesting ex-communist pistols and revolvers on the market.
Have you got a decent .22 so that you can practice your marksmanship inexpensively?
Consider attending an NRA gun-handling class.
A Hunter's Ed class will aid and abet your newfound gun fixation.
Which ought to lead to Labrador ownership.