First off, WELCOME! There are a lot of very knowledgeable folks on this forum... many of them have been in the game for years if not decades and have owned multiple brands.
Since no one else really addressed your question about the JD models you posted I will (to the best of my knowledge).
First, since you want the multiple attachments I would stay away from the JD400. I've got one and I really like it, it was JD's first venture into "Super Garden Tractor" area but they are also not as common and don't share attachments with the 420 and 430.
Back to the 318, it seems that everyone that has one thinks that the 318 hung the moon. There are a lot of them and their attachments out there but they hold their value.
The 420 and 430 are the more common super size JD's out there, the 420 is gas and the 430 is Yanmar Diesel. Both are beasts but I believe the 430 is much more desired due to it being diesel. Both have plenty of attachments, multiple hydraulic controls, big decks and more common 3pt lifts. If you are serious about gardening the 3pt lift will be very handy.
Lastly, don't dismiss diesel yet! Mowing 2.5 acres you need to consider fuel economy and durability. Diesel generally are much easier on fuel and last MANY more hours... While my neighbors burn a couple of gallons of gas per hour mowing, I burn 0.24 gallons/hour of fuel in my Kubota diesel.
Regarding the Bolens... if the funds are available and he will deal with you (that price is top of the spectrum for around here) then try it out. See if you like it. You might realize that it will do all you need or you might realize that you want multiple GT's and multiple brands around for various jobs (like most of us!!).
As far as things to check for on older used equipment. First of all if you've restored older cars you should have some idea of general condition items. Besides the engine, tires are one of the biggest expenses. If you have to replace them all then that could be $300-$500. Make sure the hydro doesn't whine too much and the fluid isn't burnt. Decks aren't cheap but rebuilding the spindles isn't too bad so just make sure the deck is solid first.
Sorry for such a long winded post. Good luck and post up what you find!