I haven't been since '98. I think I used a 28-70 and my 35mm compact most!
In so many places you can get so close to the action you don't need long lenses to fill the frame with bike too often. A lot of sections you could almost stick your hand out and touch them as they come past, you can get so close, trouble is they ent hanging around for you, and you often cant see very far up the road in either direction! By he time you see them, they've gone!
Finding a good vantage point, is probably more crucial to good shots than lenses, find one to suit your kit, rather than buying more kit to try and cover all possible vantages.
And more... cos once they close the roads? Y'stuck where y'sat! I'd be more concerned with making sure I had sandwiches and drink, and some-where to pee, and possibly something to read bfore, between, and after races, than photo equipment, TBH. Fishing stool could be useful too.
And then there's the rest of the fortnights 'spectacular' to ponder.. racing is a very small portion of that, and Douglas prom is going to give you loads of static bikes to photo, lots of candid opportunities, bands, shows, the fair, the beach-race, the Laxey trial, the Ramsey Sprint, the classic parades, etc etc etc...There is actually a point, where you are likely to think, hang on, taking these photo's I'm missing all of this!
However.. back to '98, and photo's I took then, have been scanned to digital in the last few years, and are sat on my my hard-drive in a folder called "IOM-TT-98".. waiting.. for me to find the frigging program and my note-book, in which I noted down the stuff that wasn't in the program that tell me what the heck I was pointing the camera at! Err.. funny front end, yup that must be the Yamaha GTS... Red bike.. now was that a Kawasaki or a Honda? And who was riding it? etc etc etc. On which notion... I'd suggest a pen, paper (OK, a tablet?) and a bit of organisation; making sure you don't get the program soaked in beer, and DO make notes of what you are looking at.
There's SO much to see and take in, and people to talk to, so talk to them! and try and remember what they tell you!
This Kawasaki Z1000 you are stood next to might just be yet another old Zed some-one has done a Lawson tribute job on in their garage... but on the island? That COULD actually be Lawson's bike! Or at least one ridden in the Trans-Am Challenge by one of the other aces of the day.
IoM is so much more than just a race, and if that's all you go for you are likely to miss out on so much. could almost go there and forget about the racing, there is so much else to see and do, and the 'atmosphere' and the history and the stories, that just cant be captured with a camera.. you cant convey even a tiny % of the event in pictures, let alone just pictures of the racing.
Research. Check the advance program; check the event lists; look at the maps; work out what you most want to see while you are there. Check the times, pack a little radio to be able to hear the TT Commentary and announcements, particularly the road-closures, and keep an eye on the time.. no point having found a great vantage point if you cant get to it 'cos the barriers have gone up! And time-table gets changed depending on whether, and traffic and other circumstances, so follow the commentary for the latest news...
And Don't worry too much about the photos! Just try to enjoy the event.. it is pretty awesome.