I just purchased a 03 R1 and is looking to start modifying the bike. I do a lot of street riding and was wondering if a full of slip on exhaust would be best. What is a good start.

:rock
The mods are gonna vary with the type and style of riding oyu do. For drag racing i'd lower and stretch. For an all out streetbike i would maybe lower it, but mainly exhaust, polishing heads, and other engine mods to boost hp. I've bought some parts off of Speedlink for my bike before so I'd check them out first i think their site is www.speedlink.cc but don't hold me too it

I've got a few mods done to mine (see list below). Without a doubt the best mods I ever got/done was getting the suspension re-sprung and revalved.
It overall improved comfort, control and overall made me a faster rider because of the confidence it inspired.
I'd very highly recommend it and its really not that expensive.
It's actually a pity that a piece of pipe which does very little more than make noise gets so much priority.