If they're selling it 'as-is', when you offer on the property you are essentially waiving that you are offering without any contingencies. Unless you try to submit an offer with inspection contingencies, but chances are they will immediately reject it.
Most crucial things to look for are:
- Condition of the mechanical, electrical and plumbing systems. "checking the lights" isn't going to tell you if the system is up to code, or if the electrical service is relatively up to date. Plumbing you can't really do too much other than checking the water main coming into the house and check the date on the boiler.
- Structural components - check for cracks/water in the foundation. Also in the basement, check the floor joists and supports. Check spot and record any water or mold you see anywhere in the house.
- Roof/exterior. Look at the roof, usually just by looking at it you know if it needs to be replaced. Do you see any rotting in the sofits/fascias?
I would bring someone who knows what they're doing from a construction standpoint. An inspector is the only way to be close to sure, but obviously they cost money.
Understand how much cost the rehab will be, and be aware that in order for you to be approved for the 203k, the appraised value after work done needs to be more than the loan amount. So if the repairs are more than you'll get out of the home, you will be denied.