If I were in your shoes, I would go ahead and use the Audyssey mic with the app and let it do it's thing on automatic with the range set to below 300Hz*.
When you get the Umik (and Pogre's link has it in stock), then you can use REW to provide a more accurate FR for your system than Audyssey provides. Post this and ask for interpretation/assistance in determining how much more goody you might get from using Ratbuddyssey. It really depends on your system and room. Some people get great results and see no reason to go further. Pogre got good results but decided to tweak it for even better results. Either way, we have the people here to help guide you (and Pogre is one of the best for this)!
* This is my recommendation assuming quality speakers. If you are running something like the A. Jones Pioneers (which are great for the price, but limited by the budget), then you are likely to get benefits from running Audyssey full range. I appreciate that you are not disclosing what your gear is so people will be "blind" when assessing your system's performance!