Hi Tom-
Forget about the get a series II comment for now. No one knows enough about you yet to suggest any particular level of bass.
Let's start with a few more questions:
What do you want a bass for? Thinking of converting, just something to noodle or write songs with, or doing solo recording? If it's something else, tell us.
What sort of bass lines do you expect to play? Motown-style traditional bass, or do you foresee transposing guitar licks to the bass? If the latter, you'll probably be looking for a six string, a five string tuned with a high C, or a baritone guitar instead of a bass.
Many guitarists are very comfortable with the 24.75 to 25.5 scale lengths. 34 or more feels pretty large, so you might consider a short or medium scale instrument, or even something shorter like a baritone. If you're a big guy, it might not matter, but if you're planning on bringing over some guitar lines, the shorter scale will help.
A six string bass will allow you to play well into the guitar's range. You can solo up there and it will be a bit more meaty, but similar in tone. You will also be in a good position to play cleaner 3 note chords on the upper strings to fill in behind the solos of others if it's important to your group's dynamics. On the other hand, your technique will have to be solid to avoid sloppy sounds. It's a little harder to mute the extra strings on a bass as they are spread a bit wider. Personally, I have problems playing slap lines on basses with extra strings. It's not impossible, but it takes me more effort and concentration. I'd also say that it takes some effort to set up a bass and rig to get great reproduction of notes on a low B string. A six string tuned an octave down from a guitar, or a five string with a high C instead will be more consistent in tone.
As far as sound capabilities and play feel, there is much variation. Yes, Series basses can pretty much do anything. The basses of the middle levels with Signature, Anniversary, and Europa electronics are all excellent and superior to anything made by a company not named Alembic. The lower levels and older instruments still sound good, but the drop off, IMO, is larger than going from Series to middle.
As far as play feel, there are a couple of standard neck shapes. The higher end instruments are mostly built on the standard neck taper which is relatively straight and maybe a bit wider at the nut. The other shape is the comfort taper which is narrower at the nut and wider at the bridge. You'll have to figure out which feels better to you. Also, the newer basses built over the last 10-15 years are very solid, I think more solid feeling than some of the older instruments.
With regard to amplification gear, you can play anything through anything. Don't expect to be able to get as loud with your guitar gear as you can with a guitar plugged in. Bass frequencies suck a lot more power to generate clean volume. It will take a more bass-tuned cab and some real clean power to amplify a low B string adequately. If you're not gigging with it, your gear should be fine. If you are planning to gig, you'll have to see if the gear you have can handle the frequencies you're trying to generate. I have a Peavey Classic 30 in the living room for noodling around and it's just fine at living room volume.
Now to get back to the Buy a Series comment. If you're on the used market, there are quite a few Series instruments that show up as great values. Buying new, You'll see a big price jump to the Series basses that probably won't be justified unless you're looking for perfection and willing to pay for it. Admittedly, it will cost you as much as your Further to get a great Signature instrument, so nothing new is exactly inexpensive. One other issue on the used market is that there aren't that many higher end five or six string instruments out there. You may end up having to take what you can get if you're not willing to invest six months or more in watching for the right instrument to come along.
