Without a doubt, the easiest, and cheapest way to get into WWII kit is by choosing a US Impression. Just about any equipment, uniform, or weapon is available in either original or replica. This page will link you to several US Impressions, from a very cost effective infantry impression, to an airborne impression, and finally a ranger impression.
Up to and including D-Day, the US utilized several specialized uniforms across its ground force (just before Market Garden the Army began standardizing across the theater). The most distinct difference was between the airborne and traditional ground troops. To keep things simple we won't talk about the tankers or marines right now. Variations within airborne divisions and infantry specializations were also present, but are less distinct and boil down to particular pieces of gear or uniform construction.
US Impression on the Cheap
Finally, if you want to start slow, or are lacking the funds to go hog wild on an all correct impression, Sam has put together some tips on how to buiid a decent one for about $150. Check this PAGE out to save some $$$ and still look authentic to all but the hardcore re-enactors. Make sure you are familiar with the Skirmish types if you plan on going this route. While this is fine to get you started, as the association grows, this type of impression will become less useful. For a little more money, you can pick up fully authentic repros and look 100% the part.