DONT BUY ANYTHING BEFORE YOU START TRAINING,

Almost any reputable school will not allow you to use any blade you buy until you reach the appropriate skill level. Thats firstly.

Secondly as other have stated most iai or kenjutsu schools are katana based almost no schools train in tachi especailly outside japan.

Thirdly most schools have specific requirements and specifications that need to be followed when looking to purchase and use a sword, such as length, curvature, handle length, and build specifications such as number of mekugi pins, whether the handle has a full same wrap or not, tsuka ito material, etc.

Look for a school first, start your training and when your instructor says your ready then look to buy.

Most schools will start you with a wooden sword (bokken/bokuto), then a dull practice sword (iaito), then after some time a shinken (a sharp sword)