To be honest I'd change pretty much all the consummables if it's been sitting for that long - fluids go off, as do tyres, so just to pop a list together from the above bits:
- Oil & K&N filter (10W4)
- K&N Air filter
- Coolant change (could possibly do with a flush as above)
- Brake fluid change for definite, if the hoses are the original you'd probably want to fit braided hoses as well and do it all at once - when you said the brakes are fine I'm guessing they're not binding so hopefully the caliper seals are fine
- Forks I'd definitely at the least do a service on them, change the seals for sure and the oil. if funds allow, i'd also look at getting a set of new springs matched to your weight (as that'll make a big difference on the handling front), and possibly the preload adjustable fork tops off the later models
- Change sprocket nut, and give the chain and sprockets a good clean and lube
- New tyres - definitely the rear as it'll kill the handling. Tyres that old though will probably have gone off (particularly if the bike has been sat on them) so probably the front too
- If money allows, I'd probably also look at the rear suspension, it's not amazing new (although not as bad as son) but can go off over time, so if it can be serviced (i don't own a 600 so aren't sure on that) i'd get it regassed and resprung/revalved to your weight), or if funds allow again invest in an aftermarket shock (will transform the handling, are usually adustable, start at about £300 for a Hagon and go up to thousands depending on what you want!)
Once you've done that lot I'd personally get rid of the petrol currentlyin the tank, clean the tank out and make sure it's totally empty, and drain the carbs - you can get water sat at the bottm of the tank,particularly if it's been sitting a while. Once you've done that, new petrol and some Redex in the tank and take it for a ride, and hopefully that'll remove any fuel residue in the carbs - although hopefully there won't be a lot/if any
Once all that lots done, I'd just make sure your carbs are balanced, as they tend to slip out of sync over time
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Once you've done all that, you should have a well handling and braking bike, freshly serviced that you'll be able to greatly enjoy over the summer, safe in the knowledge you won't need to do a single thing to it for a fair while
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