Having a good dealership nearby is essential and a good service or parts guy is worth his weight in gold. My service manager has walked me through countless problems on the phone and saved me thousands. On top of that, you need to locate a few good welding and hydraulic shops in your service area. I recently cracked a chrome rod on the hydraulic cylinder of my tree shear and had a hard time finding a shop to weld it up.
Before you buy a machine, check out the parts department and compare prices on hydraulic couplers, hoses and other common replacement items. Proprietary parts suck. Make sure the machine you purchase can live on a diet of cheap hydraulic oil like WalMart 10W-30. When a limb falls and breaks a coupler you will see firsthand what 41 gpm at 3300 psi looks like!
Next, check out the rental department and see if they carry the exact same machine and attachments you want to purchase or something very similar. In the highly likely event you need to take your machine in for service, its nice to know they have a rental in stock that will allow you to finish the job you were on.
My advice would be to specialize in one or two things and not be a jack of all trades. Here's my reasoning: multiple jobs take multiple attachments which require lots of different spare parts, a bigger trailer, bigger truck to pull it all, etc. Not many parts interchange between a hydraulic hammer and a Fecon mulcher. Believe me, you will buy plenty of attachments even if land clearing is the only thing you do.
I know mulchers are all the rage right now and they do have a place but don't overlook a 20" tree shear for skid steer land clearing. $8500 will get you into a brand new 20" Tree Terminator which is a very stout but mechanically simple attachment. Another $8000 will get you into a Davco rotary brush mower and allow you to tackle up to 4" stuff-anything bigger, use the shear. I don't think you can touch a new mulcher head for $16,500 and this would give you two of the toughest attachments on the market. Then, buy a set of replacement hoses and couplers for both units, grease up the zerks and get busy.
Compare replacement mulcher tooth cost with shear and mower blade cost and you will go for the shear/mower combo in a heartbeat. Also, compare tooth replacement cost between the mulcher heads you are looking at-its a significant operating expense.
Check out what your competition is running. Being the first guy on the block with a new car or boat is OK but not for a machine that you use to earn a living and especially not for a machine you are going to beat the $hit out of back in the woods. If all the old guys run Cats, its probably for a reason.
Good luck!