Must haves depends on what you have and what type of photography you do. The average basic kit of the body, 75mm lens, lens hood, waist level finder and 120 back plus a hand held meter will get you going fine. If you want one that feels and handles similar to a 35mm slr, then a non-metered, prism, or AEII or AEIII metered prism and the speed grip gets you there. A tripod really can kick it up a notch.

If you are doing closeup or macro photography the then extension tubes or auto-bellows are a must unless you can live with the 100 or 105 lenses. Talking of lenses depending on your type of photography you may find the 40 or 50mm lenses are a better standard lens having a larger field of view and something in the 100 through 180 range for portraits.

I find I use my 40mm lens more than the other lenses (40, 50, 75, 105, 250) as I tend to like a somewhat wider view than the 75 provides. A number here prefer the 50mm feeling it is slightly sharper than the 40. When I was younger my motor drive and speedgrip plus the AEII metered prism were standard fair until I decided to lighten the load and went back to the waist level finder, and droped the motor drive and speed grip. I would suggest at least the non-metered prism as standard fair along with the waist level finder but, I am used to a hand held meter.

The basic things to add to a basic kit that are used or, at least I use often are basics such as clear filters, circular polarizer (non-circular can be used), filters such as star, softening, colors for b&w, ND and others.

I also have the 2x converter so I can reach 500mm with my 250 lens. This plus the AEII plus motor drive makes for a good wildlife system.

When I got my system, I got the 35mm back and got out of my 35mm slr system so I could have both formats with 1 camera and not have duplicates. Back then a basic ETRS with WL, 75mm and 120 back was $2k. The ETRS was a bargain but not inexpensive. The only 35mm I've consistently used since was a Leica rangefinder. You can add a short zoom and long focal length zoom and shift lens, a 30mm lens, and so on and so forth; the sky is the limit but, at today's price it can be all done on a beer budget.