Toothbar for FEL is only $200 or so, Markham is most popular manufacturer.
Bucket hooks for FEL and BH are inexpensive, a thread in archives has numerous ideas. There are weld on, bolt on; I drilled holes in FEL bucket sides and put HD spring clips in when needed. Hooks also vary depending on whether it is a chain hook or slip hook (for nylon straps). Obviously chain and/or straps to go with this.
If you have the MMM, Larry @ MowerAxle has storage wheels that let you turn the deck vertically and roll it into storage. They are inexpensive.
A big funnel with filter can be helpful when fueling. I have a 17" dia. funnel that I got from a racing supply store. You also need to decide whether you're going to get 3 or so 5 gal. yellow diesel canisters by Blitz, or perhaps a "fueling station" of which N. H. Northern Tool and Supply carries several. I am currently designing/building a little boom arm which will quickly/easily clip to ROPS and support 5 gal. fuel canister while I tip and pour fuel.
Virtually all my implements are on dollies that I custom made for each one. Makes changing implements infinitely easier.
If you live where it freezes, you need to keep some extra Super UDT on hand. If it never freezes where you live, then you should keep some regular UDT on hand (both are made by Kubota and are superior hydro oil-they are the same except that the "super" has an additive to prevent gelling in cold weather-and costs more). I took a 5" funnel and stuck the small end into 2' of clear plastic tubing-this makes adding hydro fluid in that rear port so much easier. Also some extra coolant, engine oil, etc.
A "carry all" costs only $100-$200 and allows you to haul all kinds of stuff at the 3ph. I also bought a 15 gal. electric sprayer for an ATV, which I bungee strap to the carry all when I need to spray Roundup. The sprayer was only $90 vs. one with its own frame and wheels for several hundred. This more than paid for the carry all.
It was 111 degrees F this week at my house. I brush hogged while wearing a vest into which go plastic packets of chemical which freezes at 65 degrees. They stay at 65 degrees for about 3 hours while body heat melts the chemicals from solid to liquid. They really suck the excess heat out of the torso and keep the chest area at a comfortable temperature (water ice would be too cold). I keep an extra set of packets in the freezer and swap them every 2 hours. It made driving the tractor in 111 degree heat totally bearable. Gatoraide helps too.
Ear protection, eye protection, dust mask, etc. as needed, depending on how you are using the tractor.
Electric air pump (or bike pump if on a budget). You aren't going to drive the tractor to the gas station when the tire needs a little air.
It helps to have an extra pin and clip or two. Check and see if there's a TSC store (Tractor Supply Company) near you. If so, go browse around. Tisco makes a top link that's shorter than the Kubota one. Occasionally that's needed on some 3ph implements.
About 2 years ago, the "Attachments" forum had a thread entitled something like "What attachment do you use least of all and wish you'd never bought?" I'd say 95% of the responses were PHD. The tractor type will not dig in true hardpan (skid steer with correct auger will). Downpressure units and augers with Pengo carbide tips and bits can help in hard soil. If you are digging holes for planting things, your backhoe is better suited anyway. If you are putting up fence posts for a living and have no bedrock or hardpan, then it might be worth buying. Mine just sits. I would suggest that you try using just the bh for a year or two and see if that meets your needs. If you still feel you really need it, you can always get it later. OTOH, if you buy it and don't use it, it can be difficult to recoup the investment (and it takes up space).
Good luck and have fun with that new toy, er...sorry....
tool.