I don’t think you’ll need to replace a whole lot of parts; the front wheel will probably need to be trued; a shop should be able to do that for a nominal fee. Having nice, true wheels will make it easier to get your brakes dialed in.

The ‘slipping’ gears are probably due to a derailleur being slightly out of adjustment, rather than being ‘worn out’, and maybe a worn chain. Chains wear faster than you’d think, and can make a significant difference, especially on 9-sp+ indexed drivetrains.

Grips and saddles are are a very personal part of your bike, and also the things I’m most likely to change on s used bike. Fortunately they’re also pretty inexpensive; $40-50 should get you good serviceable replacements.