I think 300 used is a good deal for both buyer and seller. I've owned the 350x,550x, and the 750x and didn't have any issues with the 350x or 550x before I sold them. The 750x has been used as my head or as my power amp every show and practice since the year they came out and the only things Ive had to fix were the standby switch and I changed the pre amp tube a couple times . I would seriously consider what cabs your going to use though because you might be able to get a good deal on a 550x or 750x. With the 350 and 550 I'd try to run them at a lower ohm load. The 350 would be best for 3 8 ohm 112 cabs so the amp can run at 2.6 ohms. The 550 is the best for a 4 ohm 410. And the 750 is great for 8ohm 410s and 4ohm big cabs(412,610,810) but on the other side the 750x has enough power to ruin the cones in a 4 ohm 410 if you accidentally turn the volume up to much without realizing it. Trust me I did it.
The negative with these amps is they are very heavy for what they are. So when comparing them to say all tube heads weight wouldn't be as big a factor. Where as most solid state amps are much much lighter than most tube amps thays not the case with these (especially the 750x). If you want something lighter I'd check out the swr golight or an ampeg svt7. Another thing is the limiter sounds great if used sparingly but turning it up to much it'll cut the volume down a lot.
I cant remember if the 350x has the same ability but I run my 750x as a power amp most of the time these days by going from a separate pre into the effects return. It allows me to use the effects level and master volume as volume controls and still allows me to use the limiter on the 750x. I can run a my sans amp RBI with it for rock and metal or use it by itself for a good slap tone. So for a little extra I am able to get a lot of different tones