You're going to love the 1125! I have a Cr and I freakin love it. They tend to require a little more attention and care, but it's well worth it (up)
Things to ask/look for:
1. Stator: 1125's come stock with a pos stator that TYPICALLY, not always, only last around 5k, give or take a 1k or 2. That seems to be about average. Some last forever, but there's definitely a bad stator trend amongst these beasts. If the bike still has a warranty, HD will replace it, but they replace it with the same pos. Classic Rewind will rewind it for you and use a more heat resistant epoxy that makes it last a lot longer and gives you less worry about it crapping out on you in the future. My Cr has 2400mi on it and the stator is still good.
2. Voltage regulator. Like the stator, the stock VR's are a pos as well. This link will show you an upgrade you can do that's supposed to be a lifetime VR. You can research it and look at reviews; nothing but good things said about it and the owner, Jack, is a stand up guy and will help out anyway he can. It's an easy conversion. My VR went out 2 weeks ago and I upgraded to this thanks to RT Performance, another user, and it has been great.
http://roadstercycle.com/Shindengen FH012AA Regulator upgrade kit.htm
3. Clutch Weeping: 1125's tend to have a clutch weeping problem. I know little about this, but several people advised me make sure I ask about it before buying a bike. You can google any of these "issues" for more detail.
4. Mods: you already know this one; does it have an upgraded ECM, exhaust, air filter, tail delete? The same ol same ol. If it has an aftermarket exhaust, you'll really want to get an EBR ECM preprogrammed for it. The bikes come off the showroom floor in an "un-tuned" fashion. They still ride good, but once you put an EBR ECM on it for $250, it's a night and day difference. Here's the link to the ECM's.
http://www.erikbuellracing.com/
5. Make sure they've kept proper coolant, fluids, and lubricants in the bike. Coolant is important and these bikes tend to get warm. I just wrapped my headers yesterday to help keep some of the heat off my leg.
The rest is just the basics, man. If you're experienced with dirt bikes and atv's, you'll be just fine. It's mostly common sense. Just research and google stuff and you'll get it all figured out. I'm willing to bet money that RT Performance and Puzzled will chime in on here. They're experienced 1125ers and they've helped me out a lot. Both have great advice and know what they're talking about.
Good luck! You're going to love the ride!!!