Ive been listening to these geniuses for 35 years and apart from the utter classics, this sleeper is soooo bad ass. The drums the bass and that guitar groove at the start of this record are just sublime.
No disrespect to an Affinity (I used to have one) but thats an expensive upgrade compared to the cost of the bass itself. Tuners aren't going to make the bass more playable unless it has massive neck dive. (good strap usually does help). Tuners are basic by their nature. As long as the bass stays in tune.? I have a Gear for Music £120 P bass and the tuners rarely go out of tune..
If your going to upgrade anything on an Affinity I would spend the money on pickups. Or take it to a tech and have a conversation about what what you do and dont like about the instrument.
90's MIM Jazz with an 80's sprayed pickguard and knobs.. £300 in that condition . Difficult to tell until you play it. You need to play it and check it over. If the neck is all good and its set up right put some Aguilar single coils in it then you have a monster jazz bass. If hes your friend I would go £350.