Hi lab_owner:
All depends on what you plan to do with your land. If you are planning to keep your 40 acres in pasture for animals (horses etc.) vs. leasing out the land for hay that someone else will harvest, then I would go with a bigger machine for sure.
I have 26 acres and 14 in pasture - the rest are woods that I never do anything in. I have a L3400HST - 35hp which I think is just about the right size for me.
Most importantly you need to figure out what you want to do with your land / tractor. I am guessing that at the very least you wil be bush hogging.
My very general thoughts would be to look over 40hp at a tractor that can handle at least a 6ft bush hog. I think I would want at least 7ft.
If you don't mind gear driven vs. HST then the
L4400 is about the best deal around and should be able to handle a light/med duty bush hog like the Woods BB84 or similar.
You most DEFINATELY want a loader. Don't think twice about it. I didn't think I did until the rest here convinced me on it and am I ever glad I got it. Much better getting ths stuff upfront like a loader, remotes etc. IMHO. and working it into the deal.
Despite your question and the fact that this is a Kubota forum I would encourage you to check out all the major brands in the class of machines you end up looking at. The Kioti DK45 is also an excellent machine as are the JD4000 series. Proximity to dealers is likely to be important so bear that in mind.
Personally I had a hard time trying to judge which dealer would be good for service. None are in my area and I actually think it is hard to find out until it comes time for something other than normal service. Word of mouth is the best way IMHO. Best to go with your gut feel when you interact with the dealer. How knowledgable are they about the products and your needs? etc.
Good luck.