Hi,
I got my own first k75S about three years ago and I've learned a lot.
---Yes, get the valves set before you do any tuning.
---Check all rubber hoses for leaks and dry rot - better yet, replace them all if possible. The famous S hose, especially. I should've replaced the rubber manifolds under the throttle bodies (because the idle changed when I sprayed starter fluid on them - have a fire extinguisher ready trying this leak test!) but I got cheap and lazy and just sprayed them with Permatex rubber anti-leak spray - leak problem solved.
---Make sure the plugs and wires are correct. Stock wires are resistor. Stock plugs are NOT (no 'R' in their code). Don't use resistor plugs with resistor wires. Beemer Boneyard will sell you the correct pairing.
---Feel lucky if your inner gas tank isn't corroded. The metal brackets and fuel gauge stuff rusts and causes a real situation. Clean the tank with vinegar and seal it with fuel tank sealer because these tanks are known to get pin hole leaks even though they're alloy. If needed, Ebay fuel pumps work well for under $40 but make sure they include the fuel resistant rubber pump holder - don't use something like electric tape or it'll be eaten by the fuel. Replace all fuell hoses and clamps.
---I use the four hole injectors and have tried them back to back with freshly cleaned (Mr. Injector) stock pieces. The 4 pins are better - quicker, better response, better mileage. But they raised my idle to 1300rpm and I can't get it lower - no biggie.
---Learn the "lean drop" method of fine tuning the injection by turning the allen in the little hole plugged up atop the air box (lift the tank and slide it back a bit - the plug is in the front right top of the airbox).
---Get the throttles sync'ed (I use the Motion Pro tool) .
---NEVER turn those screws on the injection linkage that have dabs of blue paint.
---I advanced the ignition to what I guess was a couple degrees ( I loosened the advance screws under the lower engine's front panel, turned the housing a bit, rode it and repeated sequence up to the point it pinged, then backed it off. DO NOT let your motor ping doing this - serious damage! Done right, this is a solid performance improvement.
--- I just put Engine Ice in my K1 - works good so far. Whatever coolant you use, make sure it's compatible with your bike.
--- Spline lubing is the biggest maintenance topic around here. Do it right, please. If your downshifts are sticky, that's one sign the complicated front splines need lubing, but they might need it anyway and last about 20k miles. The back ones are relatively easy and should be done with every tire change. Read up on this!
---Brake fluid - use DOT 4 only.
---I've read up on the oils and have concluded; Mobil 1 for the motor is fine, and any other more expensive oil will give no noticeable improvement. Gearbox oil is another matter; I used Royal Purple synthetic 75w-90 in my K75 and it improved whatever the previous owner's dealer used - better shifting for sure. But in my newly acquired K1, I just switched to Liqui Moly 75w-140 - even better! The K75 gets moly next. In the final drive I read it's best not to use synthetics, just go with o.e.m. recommendations. And NEVER use any moly stuff in the motor - it will screw up your starter sprag because it's so slippery. You'll be left without a starter and lots of trouble getting the moly out again.
--- Fork oil - first, make sure you know what fork you have. Earlier K75 forks take less oil. When I first got my K75, the front was bouncing and diving way too much. I drained the oil and carefully measured what came out of each leg. The P.O.'s dealer put in the exact correct amount... but for the EARLIER bikes, not my '93! BTW, I refilled them correctly with 7wt (stock is 5wt) , and it was better than before because of the level, but it kinda felt like it was 'choking' as if the rebound and compression circuits couldn't handle the thicker oil - and I'm 225 lbs. Next change I'm using 5wt. If I need to tame any fork dive, I'll add oil a tablespoon at a time (as told to me by a renowned suspension expert).
---If you're looking for a lightweight battery and you want to go with lithium (two pounds or less), be very careful. Do research. I used one in my Suzuki racebike and if it wasn't almost fully charge during down time, it drained to nothing and never came back. Don't believe the claims about them coming back from only one volt, just google lithium complaints. However... Battery Tender company, which sold an earlier lithium battery which they've since halted, has on their website a new battery about to be announced as I write this. I'm watching for it 'cause my K1 needs to drop a few pounds and I trust them more than others.
Good Luck!