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Don't get all hung up on the valves. Lots of guys are still making a living on "traditional" rotors. Just find a horn you like and learn it. Bass trombone is a very different animal from tuba, don't believe anyone who tells you otherwise. I would strongly recommend lessons from a trombone teacher. Also, don't think you have to get independent valves, many pros still play (and prefer) dependents, and some still play single valves. Don't let an independent second valve be a substitute for poor slide technique.
When you have mastered the instrument (think years, not months), then you can worry about the valves, and the custom horns with dozens of bells to choose that sound 1% different from each other.
The Getzen 1052 and 1062 are good "middle of the road" starting points if you are looking for a new or nearly new horn, and many pros continue to play them. The 1062 (formerly known as the David Taylor model) is designed to to have one foot in the classical camp and one in the commercial, the 1052 is slightly more commercial leaning but still very versatile.

P.S. Please use a "real" bass trombone mouthpiece if you actually want the horn to sound like a bass trombone. The Bach 1 1/2 or 1 1/4 and the Schilke 59 are good starting points (also the Griego 1 or .75)
Start with something too big and you won't have a high register or any clarity and center to the sound. Don't try to be Charlie Vernon your first (or second or third) year.

Being a bass trombone doubler myself, this world is a viable double to have. Here are somethings to consider:

- you must be able to play the bass trombone big band chair.
There is a TON of money out there to be had in the big band. Whether it is a pickup band for a function, or a steady gig - these are out there. Don't worry about the 60K Orch. gig, all of those players (Bollinger, Yeo, Sanders, Vernon) are there to stay for the next 10 years AT LEAST! Learn and work on your jazz (it pays my rent even today).

- try out every horn you possibly can.
The Midwest Clinic is a great place to try out horns. Every year in Chicago - there are tons of instruments to try. I would recommend looking at Edwards, Bach and Shires. I perform on a Shires Dependent Twin-Thayer with a retro-Conn style bell (like the old 62H horns). It is a snappy, commercial sound, and it is great.

- don't get hung up in dep. vs. ind. valves.
Basically, this is the difference.

DEPENDENT
You have to have the 1st valve depressed to add the tubing of the second valve. CON - You lose all of the "2nd-valve-by-itself" fingerings. These can be important, but they are not necessary. PRO - You only have to "blow" through one valve unless you are in the low range. This is very important - it makes for a FREE BLOWING HORN.

INDEPENDENT
The 1st (F) valve and the 2nd (Gb) valve work independently of each other, and when pressed together, will give you a low D in first position. CON - You are always blowing through 2 valves, whether you use the 2nd valve or not, you are always blowing through it. PRO - there are even that many more slide and valve combinations.

Basically, you need to try these out. For people that think dependent valves are dead, Doug Yeo www.yeodoug.com performs on a dependent Yamaha horn. Yeo is the bass trombonist with the Boston Symphony, so he might know a thing or two.

- practice you double, everyday.
There are way too many musicians out there just to have something that, "I can play if I have to." If you expect to be a viable doubler, you have to put in the time. There are way too many good musicians to have a crap double. If you don't have the time to give, stick to one baby.

- the tuba/bass trombone double makes a lot of money.
Almost every broadway musical has a tuba/bass trombone double book. These shows come through town and they hire players. I make a living do this, it is not hard. The parts are easy, and they are fun to play. If you are a tuba player, it is great to have some experience on the horn.

I have enjoyed my Holton TR-181 for many years. It has dual independant rotors, produces a very centered but large sound. They are very affordable and are very good for both orchestral and commercial playing. Its a great all around horn and can be had for a reasonable price. I have found it works well with a Marcinkiewicz 105 Phil Teele mouthpiece.