something old with a shaft (I do the same mileage (roughly)) - it equates to 550 miles a week, 2,200+ a month.
GT550
XJ900 (shaftie)
Bros 650
You don't want to go smaller than 500cc imho cos otherwise you'll be ragging it
everywhere and it'll get bloody tiring. Factor in "time" - you'll be spending 3+ hours
at least in the saddle
every day so make sure the bike is comfy, but fun, otherwise you'll end up hating it.
Also think about buying clothing that is intrinsically waterproof. One pieces are fine for occasionaly use (I have one for utter downpours), but if you're struggling into it daily it becomes far more hassle than you'd imagine, Look at Hein Gericke stuff - especially the GoreTex ranges. Get clothing that's not too warm in summer, but has a liner for winter. Do NOT buy it tight - you'll want to get a jersey underneath on those really cold mornings.

ALso - do NOT buy the clothing off the rack - ask the salesman for something fresh out of the box (it won't have been tried on by 30 different people and the seams won't leak

)
See if you can get hold of a WindStopper fleece - they're FAR thinner than normal fleeces but just as warm - makes a BIG difference because they're not bulky.
Also factor in the costs of maintenance. You may well find that a season ticket is equal, or
LESS cost than the overall cost of bike, clothing, insurance, maintenance, fuel, tyres etc.
Mine certainly is over a year - by about £500 - BUT I enjoy my ride to work every morning and the ride home.

I get to work awake and fresh, as opposed to half asleep and bored after a train journey, and I can time my arrival FAR better than I can on the train. The ride home allows me to forget work, and release any frustrations.
Great Prophet of Veedism.
non quod sed quomodo"BBQ fluid is a sweaty one night stand compared to the long term relationship of a properly burning wood fire."
Cool in a fuddy-duddy old fart kinda way -
Wingnut