1. Find out how files are to be transferred. The most likely scenario would be a secure FTP site. But this needs to be verified.
2. Get any documentation advancedmd may have on the HL7 files they will send and accept. Get some sample files, also. Especially need to know what version of HL7 they are using.
3. Cost to implement from AdvancedMD. Yes, although your developer will be doing all the work, advancedmd will probably still nail you with something. That's how the online guys roll. Because they can.
4. Realize that a custom interface may not solve everything. If you need some piece of information and advancedmd is NOT sending it in their file, you're SOL unless advancedmd is willing to do some work for you which will cost $$$. Also, the reverse is true. If you need advancedmd to receive some information you can send it since you can just tell your dev "send this" but if advancedmd doesn't read it then you're still SOL.
JamesNT