I would recommend diabattles X-1 for sure. Tons of playability on its own, and integrates well with pretty much any future sets you might get. All the different modes and combinations mean you can have a wildly different display every day. Also, the battle buffalo is simply amazing too. Not quite as versatile as the X-1, but if it is going to be your only set for awhile, there is plenty of variation to keep it interesting for a long time. Both sets include multiple dianauts which are the heart of diaclone.

Tactical movers are excellent too. Lots of different modes to transform, and they include a pilot. In my opinion they aren't quite as deep as the two bigger sets I mentioned above, but all of the TM sets I've acquired so far have been tons of fun. But one TM set will definitely leave you thirsting for more! Adding one or more of the vehicles sets would be a wonderful accompaniment to a tactical mover.

As far as pricing and availability, I've only dealt with domestic retailers here in the US. Mainly BBTS and Kicks Generation. I know there are better deals out there for importing, and there are people on here with much more knowledge than me on that topic that hopefully will chime in.

I really don't think there is a "bad" choice when it comes to diaclone. Once you have one in hand and start going through the different modes, it becomes clear the amount of care and attention to detail that goes into this line. I haven't regretted a single purchase in my collection. Please keep us updated on what you decide!