If I were you I'd dismiss the UC100 and would go for the UC400ETH. The only advantage of using UC100 is that it is a "plug-an-play" solution and the motion controller is boxed, ready for immediate use. With the UC100 you will need an external power supply for the motion controller, and a PC with a free Ethernet port, but it has many advantages over the UC100.

Regardless of which, you have the option to buy a cheap desktop with parallel port and 32 bits Windows 7, which is the last version with parallel port support. Basically, this is the cheapest solution. If you decide to buy a motion controller from CNC Drive then you should buy a UCCNC license as well. It does not cost a lot extra, so you can take a risk with it. If you don't like it you can always use Mach3 with the CNC Drive motion controller. UCCNC works with almost any PC, and since it is using an external motion controller, it works under XP, W7, W10 32 and 64 bit OS. You can also download an evaluation version, which has all the functions except motion, so you can test and see if you like it.

Of course, Linux is always there, free of charge, but that option depends on how computer minded you are, I don't think it is as simple as UCCNC, but you can use it 100% free of charge if you are using the parallel port.

Personally I went from Mach3 and UC300USB to UCCNC and UC300ETH and never really looked back, except that today I'd buy the UC400ETH instead. In any case, you have to look at your own way of using Mach3, all G-code dialects are a bit different, so the decision depends on how familiar you are with programming, and how critical the machine is for you. Some functions which Mach3 has are not supported by UCCNC, some functions may need some different approach and so on. I believe that is the same with Linux as well. In other words, it is simplest and cheapest to continue using Mach3 if it works for you, because it just needs a new PC, which you would need to buy regardless of the alternatives if you think it is near it's end of life.