Leaving it at 3v makes not one bit of difference.
It doesn't matter whether you are in variable wattage mode or variable voltage mode, when you make adjustments, you are varying the volts applied to the coil. The difference is that in variable wattage mode, the device is calculating the voltage to apply by taking into consideration the resistance of the coil. The end result being that if you change coils to one of a different resistance, the new coil will hit exactly the same as the old coil, without having to make any further adjustments.
So a variable wattage device is more of a "set and forget" type mod. Of course, in practice, it doesn't really work like that. For example, dual coil set-ups prefer higher watts than single coil set-ups. Home made coil / wicks can handle more power than pre-made due to wicking, and so on.
Now, as to whether you should go for a Vamo or not, £55 is a lot of money. No two ways about it. The bonus with this vaping malarky is that second hand items tend to sell for almost new prices. Although that wont apply to the likes of the CE4 clearos, because they are poop. So you will unlikely lose too much money if it isn't for you.
That being said, you still have to shell it out in the first place. Rather than forking out for that VE starter kit, think smaller still. I started out with something like this :
http://www.paradisevapeuk.co.uk/product-p/express.htm
At only £8, the same price as a packet of cigs, it was a fine little introduction, and will leave you with a back-up device if you do decide to go more advanced.
Add to that a couple of flavours, from there they are about £3 each, and you are good to go. I started with Irn Bru and Mojito, and they were both excellent from there. There are nicer flavours elsewhere, but then you have to pay extra postage and stuff, so can get quite expensive. At least if you buy everything initially from the same place, you are cutting costs a little.
So you could get started for as little as £15 including e-liquid and devices. If it's not for you, then not a huge amount lost. If you feel it could work for you, then Vamo / Protank is a good way to go. Although I'd be pretty tempted by the MVP from myepack as you don't need to be messing with batteries and chargers, the internal battery lasts a couple days, and it's a nice wee size.