Easy one!
1) You must pass an instrument proficiency check as per
FAR 61.57 , administered by an authorized instructor in the same category of aircraft (airplane) that you seek instrument currency in.
2) You must have at least a 2nd class medical in order to act as a required flight crew member for compensation or hire (exersizing the privileges of a commercial pilot) as per
FAR 61.23.
3) If you have not had one since the ATP checkride, your biennial flight review currency will expire on March 31, 2006 as per
FAR 61.56.
So I suggest the following to most efficiently serve your needs for the near future:
1) Obtain a new first class medical if you can. It doesn't cost any more than a second unless you are required to have an EKG due to age. (additional $50.00 or so)
2) Hire a CFI-I to administer an IPC and a BFR. Most, if not all elements of the BFR can be done concurrent with the IPC anyway. To control cost, this may be done in a fixed-gear single. Niether endorsement requires any specific training before the check. When proficiency has been demonstrated in all required areas, the CFI may issue the endorsement. (progressive training and checking) This is not a flight test for a certificate or rating! Train to proficiency and move on. The requirements for the BFR are simple and contained in the text of FAR 61.56. The requirements for the IPC are found in the rating task table in the instrument PTS. Here is a link to the 2004 version for reference.
Big PDF file!
After the IPC and BFR, you will only have to meet takeoff and landing currency in class (if other than the class of airplane you do the IPC/BFR in) if passengers are carried. That takes care of the FAA regulatory requirements. Insurance coverage and taxable income considerations are separate issues!
See? I told you it was easy!
Hope this helps
Best regards,
Westhawk