I may still be a memeber of the "bigger is better" school...but let me ask...
How big are those couple lots? Are they in addition to the five acres? How quick do you need to get the work done?
Worst case it sounds like three septic systems, three driveways, and a bit of landscaping.
You don't need a huge tractor or deidicated TLB for those tasks, unless you want to finish them super quick. Even then, you might be better off hiring someone to do the heavywork and buying a smaller tractor for the long term.
My gut says something along the lines of the Kubota
L3130 or 3430 would be enough tractor, if you put the BH90 backhoe on it. Or any color in the similar size.
I have a
B2910 that is ideal for the heavy work maintaining/improving 3.7 acres. And a smaller one for light duties and grass cutting. My macho side wants me to get a bigger tractor, but my practical side knows that a bigger tractor would not work as good for me as it would not fit between the trees, and so on.
You need to balance the immediate with the long term. Sounds like the immediate would like something on the large side, but the immediate will not last that long...once you transition to the long term maintenance of the property side, you may find that the tractor that worked so well in the beginning does not quite suit the tasks at hand.
I guess what I am trying to say is, I think it may be better to suffer a little in the beginning, and be happy in the long run. Bigger is not always better...
Now if money were not an issue, I would say buy big first, sell it or keep it, and buy smaller later...have your cake and eat it too! Generally money is an issue though...