Depends on your risk level. If you just want to find out what 5V vaping is all about, it's about like what a 3.7V is with a 1.5ohm carto. If you know you like that, you can get a straight 5V PV, or a VV. If you're on the fence, you can try a VV like the Lavatube. They're going pretty cheap and you can get a full kit for $59, or one of the newer ones that'll let you up run a lower resistance carto at a higher voltage for $69. Or, if you're confident that a VV is in your future, you can jump to a Provari. There is also the Vmax, which is between the Provari and the Lavatube in price but, in some respects, outperforms either one. Keep in mind that it's a new product, so not as proven as far as durability and long-term performance.
I wouldn't stack batteries in the Bolt. They have a cheapo switch and are basically a cheap knock-off of the king of fixed voltage PV's, the Silver Bullet. By the time you get a Bolt, batteries, etc., you're not even saving much money. Generally, I wouldn't recommend stacking batteries at all, although I do it myself in my BB, which is a smaller version of the Silver Bullet. You especially don't want to stack batteries in a cheap mod.
The other thing you can try is one of the cheaper "selectable" voltage mods. I think one of the eGo types will let you choose between voltages, although I'm not sure if they ever reach a true 5V.
If you use a reasonable voltage, like 3.7 to 5V or so, and you use standard resistance cartos, there isn't all that much difference in the vape quality between a Provari and a Lavatube. The Provari is built better, it lets you use a slightly lower resistance at higher voltage than the newer Lavatube (warmer vape), and it's less susceptible to voltage drop-off as your battery drains. The newer Lavatube may, or may not, address that last issue. It comes down to aesthetics, long term build quality and if it's worth the extra money to you. Worst case scenario is that if you get a Provari or, to a lesser extent a Lavatube, if you don't like it you can sell it and get a good amount of your money back.
The Reo is a whole different animal. You can always sell your Reo for close to what you paid. Maybe you should try the best of both worlds and get a Reo and a Lavatube. If you like VV, you can upgrade to a Provari or Vmax (if the long term reviews are good). If you don't like VV, you can keep them both, or sell the Lavatube, and not be out a a lot of money. Just because something is VV, it doesn't mean you have to vary the voltage. In general, it's always good to be able to vary the voltage, even if you don't ever vape more than 5V. If nothing else, you'll never need to keep track of different resistance cartos or attys. Just stick with 3ohms, and you'll be golden no matter what juice you like or how warm a vape you want.
