I have to replace the entire cockpit on my Cervelo P2c - the 8 year old aerobar and stem has rusted out pretty well after getting sweat on a ton, and then being neglected for the past 5-6 months while I've been a pure marathon runner (it sat in my garage and the condensation apparently made all the bolts rust out,and it's time to change it out anyway.)

I'll need at least:
- STEM
- AEROBAR (including base bar)

There are good videos online on how to change these things out - I'm sure it's a lot trickier than it looks, but I'm willing to give it a try and then when I totally fail, go to the LBS to have them bail me out.

I however have no idea what to look for size-wise when buying these things. Are all stems the same? Are the road vs triathlon bike stems?

And with aerobars, there are setups where they sell both the basebar+aerobars in an kit (I prefer those), but a lot more varieties of basebars sold separately from the aerobars themselves, which may give more flexibility, however I have no idea what sizes to match up with.

Can anyone tell me the basic specs/sizes or any major caveats when buying these parts other than picking road-bike type parts? Are there any major size differences that cause pitfalls when mixing/matching parts?

I'm a total noob at bike wrenching, so excuse the ignorance, but I'm a willing student, and I've got time to fix the bike over a long period of time since I'm not racing on the bike any time soon (marathon running for now.)