For the money you want to spend, a Mesa Nomad 45 1x12 combo is an excellent tube amp. It has the most tonal versatility with 3 channels(Clean, Classic High Gain, and Modern High Gain). All three channels have a swithch that can change the gain structure of that channel, so basically, you have six different channels to choose from, with 3 of them foot-switchable at a time. Also has a "solo" button on the footswitch to boost your volume during leads, which is a very handy tool.
If you are new to tube amps, there are a few things you need to know. Tube amps are pretty reliable, as long as you don't let your tubes get in bad shape. If you play with bad tubes for extended periods of time, you can blow output transformers, ruin capacitors, and do all kinds of things to your amp, none of them good. Usually, replacing your tubes every 9 months to a year will ensure your tubes are in good shape, although, you may get a bad tube from time to time.
Another thing is, if a tube goes bad on you at a gig, you are pretty much done playing on that amp for the night, so a back-up amp is a pretty good idea.
Tube amps sound their best when they are being pushed at higher volumes. I'm not saying they sound bad on 2 or 3, but if you want the most out of them, you have to turn them up. This is something to keep in mind when choosing the size of your amp (amount of Watts). A 30 watt amp is generally loud enough to gig with. The difference between a 50 and 100 watt amp is only about 3 decibels, so you are paying a lot of extra cash for 3db if you go with a 100 watt.
Tube amps run a lot better when they are hot, so they work better for outdoor stuff in the summer, where some solid state amps get too hot and shut themselves off.
Don't forget, speakers make a big difference in an amp, so when amp shopping, try an amp through a couple different speaker cabinets before you decide not to get it.