there are loads of old automags out there, that can be picked up for a few quid, for a bit more money, there's old angels, and loads of old autocockers out there too sitting in people's kit bags.
I bought an evolution Autococker a year ago for a meer £350 from Bully, and it shot fantastic, strait off CO2 and was perfect at entry level tournament.
Tippmans can be used at a tourney level, as proven by Tippman Effect who recently won big in the states, however the guns do need a little extra love to get them up to spec. The same thing with Spyders and Bad Company, they use low end guns but trick them out to get better results.
You can shoot whatever you're comfortable with, but you may find that there's guns out there that are better suited for Tournament paintball than your Tippman. Do you know any tourney players personally? if so ask about, many of them have old markers that they'd be willing to sell you that would be perfect for tourney paintball. I think it's how a lot of us got started.