liquid ballast or not. it completely depends on what you will be doing with the tractor.
give some details about what make/model of tractor you are looking at, along with what you will be doing with the tractor...
examples does it have a FEL (front end loader), 4x4 or MFWD, are you on any sort of hills? mowing the lawn (directly around the house) or mowing as in pasture with a bush hog? doing any major pulling with the 3pt hitch. for field work? or a garden? moving dirt around? what type of attachments 3pt, bellow, or for the FEL are you looking at?
i already went through the hassle of getting stuff around to deal with filling up tires with liquid. so when i do get a leak, from a thorn, nail, stick or what ever might of caused a leak in tire. i can drain the liquid out into some buckets, deal with the leak, and then put the fluid back into the tires on my own. DIY fluid filling of tires is fairly easy. if you are a DIY type of person. a coupe houses, a generic 12v pump, a fuse, a dip switch, some battle cable clamps, (to attach to battery in a vehicle) and some wire. and you are ready set go...
as far as fluid, i say go with something you can easily get at any given time. vs hitting specialized shops, or items that may need to be special order. RV antifreeze for example. fairly easy to get walmart and menards and other major hardware stores carries the stuff. and if you just fixed a quick leak in a tire. a quick top off of fluid could be had.
tube vs tubeless. prefer tubes myself. little more rubber something has to poke through to cause a leak. and i run the tires around enough stuff, that my luck i would nock a tire bead loose from edge of rim. and have a flat if i didn't have a tube in it. ... that and when i do have a leak with fluid in the rear tires. majorty of the liquid stays in the tube. well i should say so far. my luck next leak will tear entire gash into a side wall of a tire, and loose all liquid...