I am also a big fan of the Dyna ST70, but I prefer it stock. It definately has its flaws, and many of the popular mods improve some of these flaws quite a bit. But none of them really transform it into a super-amplifier, and the original design is a masterful work of simplicity and careful compromise. I recommend that you give the original circuit a chance first, so you then have a good understanding of what the modifications are doing.
And while it may push your budget a bit, you should look for McIntoshes as well. You might find an MC240 that sounds great but is cosmetically flawed for pretty close to your price range. They are much more stable, consistent, and reliable than Dynacos, they use extremely common power tubes (6L6s), and they operate the circuit in such a manner that the tubes last much longer as well. Oh and the sonic performance is a HUGE step over a Dyna.
And while it may push your budget a bit, you should look for McIntoshes as well. You might find an MC240 that sounds great but is cosmetically flawed for pretty close to your price range. They are much more stable, consistent, and reliable than Dynacos, they use extremely common power tubes (6L6s), and they operate the circuit in such a manner that the tubes last much longer as well. Oh and the sonic performance is a HUGE step over a Dyna.