I really don't need a Lion battery, and a lead acid will get me the modest distances and power I am looking for. (It used to be, anyway, and the kits are better now, it would be heavier and larger than a small LiIon though)...
BUT -- do you think the reduced cost of a lead acid versus LiIon is a frugal (value) or a cheap choice? I have not closely tracked costs / power ratios, but would love to hear anyone's thoughts.
Don't buy a lead acid battery for an electric bike. They're dreadfully heavy, put out a good bit less power than you'd expect, and have a lifecycle measured in single digit months if you actually use more than about 20% of their capacity. They're false savings, and you should not buy a lead acid battery for an electric bike. If you get one that's big enough to be useful, you stand a good chance of cracking your frame (I know someone who learned this the hard way).
Get a small lithium battery if you must, but get a lithium battery.
I think some of the better kits out in my opinion are bionx and the erad bbshd,
I fully support people buying BionX. I make very good money rebuilding their batteries after they've died and BionX has left the owners out to dry. It's not a good choice if you care about money, though. They're entirely proprietary, and people who spend their time and energy reverse engineering them don't tend to sell the results cheaply.
a cheap option for batteries is making your own with lipo packs from hobbyking lipos, there are some YouTube videos that can show you how. Some people worry about using LiPos as they can catch on fire or something, so you'll want to have a non-flammable place to store it.
And ignore this advice. Hobby lipo packs are a fireball waiting to happen. They're great, if you want to build a super high power Pike's Peak racer or something, but there is literally zero reason to use them for a daily ridden bike. They require careful charging and maintenance, and those packs don't have a battery management system most of the time - the BMS will do things like "keep you from damaging your pack" and "allow you to plug a regular charger in instead of having to fiddle with micro connectors daily for charging."
Over on Endless Sphere, there are "perpetual battery thermal event" threads and such, for people to document when their damned lipo packs catch fire and try to burn down the house. It's stupid.
Any of the Luna Cycle kits should work fine - they sell good stuff. I just haven't built a new kit recently, and the next one I build is going to be a 30mph highway cruiser, so not really relevant to most people.