To start Adam, at the bare minimum, you will need a tunic (Linen or Wool are the most authentic), a military belt called a "balteus", military boots/sandals called "caligae", as well as a sword, and some means to wear it.
Other gear that will come in handy as you go (to get what people will immediately recognize as a Roman legionary) will be the helmet and armor, lorica sementata (plate armor) and lorica hamata (chainmail) are the most common.
A shield called a scutum and a throwing spear called a pila are in my opinion the finishing touches.
I have just gotten into doing reenactment impressions this year, and 6 months after I began, I feel relatively happy with my kit, though I still am in need of the pila and scutum.
You can check out my impression, as well as many others in this link (mine is near the bottom of the page):
http://romanarmytalk.com/rat.html?func=v...start=2680
That is mid first century impression, just about all gear is different depending on what era you want to portray. Don't make mistakes that myself as well as several others have done and buy cheap items thinking "it looks the part" as a fellow reenactor, I'm sure you realize how hazardous that can be. Many of us are happy to be consulted with what equipment is appropriate. Expect to spend about $500 for a frugal store bought kit, though nearly everything we have can be made by hand if you have the tools, time, and some skill (though not much skill I'm told, most of us are 21st Century humans with no prior experience on this stuff). Feel free to ask away...