First time Ebike

Looks like you are on the right track. On the subject of batteries, yes it is worth it, unless you are just into the tinkering part of it. Lots of us are having good experiences with Lifepo4 batteries bought from an ebay vendor called ping. Other verdors of identical looking batteries have had issues with delivery. Personally, my ebike could not perform to my needs for range untill I had a 20ah battery. If you need to ride far, or have hills, get the biggest battery you can afford. You should be able to buy a 20 ah from ping for a similar price as the 12ah nimh. If you email him direct, you can save money over bidding on ebay. ping just uses ebay for ad purposes.

I have had excellent luck with these guys: http://www.ebikes.ca/. The website is a little rough, but they run their business very professionally.

Are you asking if it is worthwhile building your own pack? I guess it depends on how handy you are with a soldering iron. I've taken a dead pack apart before to see how it was done and I know I don't have the skills to put one together. Soldering the tabs on the batteries without overheating them is a real trick. That and you're dealing with enough electricity to fry yourself crispy. Also I have no idea where they get than giant head shrink tubing that it takes to hold the pack together.