Have just fitted a 4.5 x 18 3 spoke rim off a 2000 R1100RT with a new Pirelli Angel GT 160/60 R18. I left the original wheel/diff spacer on and have about 3mm clearance on the right side of the tyre to the closest part of the swing arm - I can live with that. The clearance on the left side is good due to earlier mods I did to the exhaust system. I have replaced the original muffler and collector with a BKSSpeedworks collector (beautifully made and compact) and a Delkevic oval carbon fibre silencer/muffler. The muffler extremely light and slightly upswept. I made an aluminium spacer to fix the muffler body to the left footpegs bracket.
Plans for the front end include upgrading brakes but retaining original forks. Have a 20mm (K1100) front master cylinder in transit and will fit some mounting brackets I machined so I can fit a pair of new Brembo P4 30/34 C 4 pot callipers. I decided against changing to a non-BMW master cylinder because it requires fitting of non original switchgear which creates a series of electrical problems to be solved. I have a 3 spoke BMW 3.50 x 17 from rim and new radial tyre. Will look in to practicality of fitting them to the K100 front end.  If too difficult will either source a K1100 front end or keep it for a 1995 Suzuki GSXR1100 front end I have as a option for a part restored 1985 K100RS. 
Plenty more to do - Solve the cut out when hot problem - looking at Hall Effect Sensor - thermostat - Coolant Temperature Sensor - Valve clearances.
Will try to hook up with the Northern Tablelands gathering in November - I'm 14 km from Uralla these days.

You have a 3.5 front 17"? three spoke. I thought it would have been an 18"?? So what size tyre does that allow? Would it run a 120??

Two Wheels Better is correct about the front tyre size for the 17" wheel. My plan is to fit the 17"  front 3 spoke rim to a 1995 GXS1100R front end I have. This at a later stage will go on the other K100 - a 1985 model. For the 1983 K100 I'll need to look into a suitable 18" front rim able to take a radial tyre and suitable for the K100 1980 front end - otherwise I'll have to consider the K1100 front end. Time for more research.

Wasn't the stock GSX1100 front wheel a three spoke? That would match the R1100 rear in looks.

K75cster you are correct. At the time of purchasing the 3 spoke front BMW wheel rim I was obsessing about trying to match the front wheel with the newly acquired rear 3 spoke BMW wheel. I'll source the 3 spoke GSXR1100 front wheel. This will make the process of fitting the GSXR wheel to the GSXR forks far easier, quicker and cheaper. I already have 6 pot callipers, suitable new discs and front mudguard. A further advantage is that this will leave me with 2 complete K100 front ends - one in situ on one bike and one ready to fit/or as a parts source. 
I'll proceed with the "83 machine first and fit the 20mm master cylinder, do the K1100 throttle system modifications (throttle handle, cable and judicious bending/fitting) and then fit the gold 4 pot Brembo callipers with my custom calliper mounting plate. I'm going to stick with the original BMW rim and discs because my adapter plate permits the 4 pot callipers to be correctly positioned to the disc surface. 
At a later stage will fit the GSXR1100 front end to the "85 machine. Two issues to address here - modification of the steering stem for a good fit and possibly needing to move the tank back a bit to allow for the GSXR fork on full lock.