There are few early decisions that will lead you to your kit manufacturer.
You mentioned the desire to build and the price range, that's a good start. That rules out the turn key options/manufactures, like Superformance or Backdraft.
Do you want to build from a donor? That is, are you planning to use a 5.0 Mustang for the engine/transmission/suspension/... If so than take a good look at the Factory Five or B & B. There are others.
If you don't want to go the donor car route, but actually purchase the parts new on your own, or through the manufacturer, then you are looking at the likes of ERA, Unique, Shell Valley...
Keep in mind there is overlap. You could build that Factory Five with all new parts...The possibilities can seem endless.
Look at the kits. What do they come with, or more importantly what they don't. I actually sat down and made a check list to compare the apples with the oranges to get a better idea what was right for me.
Make the calls and talk to the shops, stop by if possible, look to the guys in your area to see examples of finished products, and get the build manuals. Do a search here for builders, someone put together the list on a map. Great visual as you plan for that summer vacation.
Getting the manuals was a big help to sort out just what was required parts/$ wise and how much guidence/help I might need (some are much more detailed than others, to me that made a big difference). Yes, they cost a few bucks more than a catalog but for me it was a big help. To spend a couple hundred to avoid a couple thousand dollar mistake or unfinished project, is worth it. Besides, when read before bed it makes for great dreams.
For me, I wanted a 289FIA to be a little different. That meant a small block 289/302/351. I went with a 302. That limited the manufacturer to Unique, ERA or Kirkham. Will go with ERA, great feedback, thorough/detailed manual, nice shop/people (hit it on the way to see the in-laws in NH), no donor, and fit my budget.
I think the best kit is the one you own. Tell us more as you go through your search and ask alot of questions/do a lot of searches on this site.
-John