Not sure about the voucher, is it for a bike shop or department store?
You best bet is to find a trusted local bike shop. Explain what kind of riding you'd like to do, what your budget is, what you didn't like about the last bike, etc. Most are incredibly helpful, if not, then find a different shop. Oh...and go on a Tuesday morning, not Saturday afternoon.
As far as loosing weight, much has been written, but it seems that input is way more important than output. For example, I just returned from a 6 day, 320 mile, fully loaded tour. I only gained a pound.