First of all lets define an Ultralight
1) Weighs less than 254 pounds empty weight, excluding floats and safety devices which are intended for deployment in a potentially catastrophic situation;
(2) Has a fuel capacity not exceeding 5 U.S. gallons;
(3) Is not capable of more than 55 knots calibrated airspeed at full power in level flight; and
(4) Has a power-off stall speed which does not exceed 24 knots calibrated airspeed.
I think what your talking about is an Experimental aircraft which is a whole another ball of wax. What do you intend to do with this airplane? There are homebuilts that will do 400 knots at 28,000ft...Speed directly equals how much money you want to spend when it comes to Airplanes. Are you a pilot? You can fly a true Part 103 Ultralight with no PIlots License but I doubt 55kts is what you were meaning when you said good speed. To fly an Experimental you need to hold at a minimum a sport pilot certificate.
Here is a good link to the Homebuilders Directory. Generally FAST and STOL do not fall into the same sentence. While there are some that cover both realms generally it's either one or the other not both. http://www.homebuilt.org/directory/directory.html
It sounds like you have not done very much research yet. My advice would be to check out your local EAA (Experimental Aircraft Association) Chapter and get familiar with what is out there.
Here is a link where you can put in your location and find the nearest Chapter to you. http://www.eaa.org/chapters/locator/