While the cats right, so is Darkstorm IMO. You really should have a preference of strings by now. Just my opinion, but choosing your strings should come wayyy before choosing/building a custom bass. If you put the wrong strings (as in ones you dont like) on that 7 string, it will sound awful to you. A big part of tone comes from the strings, and no matter how great the bass is, it wont change the strings. I build too, and find knowing the how the strings sound on different basses a big help
Not meaning to sound harsh, but your description of the strings you would like isnt going to help much. My advice (as annoying as it may be

) is to buy a few sets for a four string and see which you like. Some will vary massively in tone. If you dont have a four string, you could buy a real cheap one, it will probably still work out cheaper than experimenting with 7 string sets

. There arent really any shortcuts in finding the perfect set (for you).
Hope this helps a little, and hope I'm not sounding offensive in any way.