I'm fairly new to e-biking... I started with buying a used recumbent bike last February. Enjoyed riding it, but found hills (heh, "hills" in coastal FL) to be no fun, and felt that I'd enjoy riding more with just a little more speed. BUT, I'm riding for exercise, so I still want to actively pedal.
Long story short: after much research, I bought a TSDZ2 kit, installed it with a home-wired battery kit, and couldn't be happier with it!
It is a "mid-drive" (crank-mounted) unit with torque sensing that basically amplifies whatever input you put on the pedals. The controller is extremely adjustable to suit your needs. I've got mine set up with 4 levels (it can do 10) and I ride on level 2, which basically multiplies my torque by 50%. I have level 1, which is something like 30%. Level 3 is fully double my input. And if I want "all of it", that's level 4.
It's a 750W motor and I'm running it at 54 volts (when fully charged, it drops to 50-52 pretty quickly). So, on my Level 2, I find that cruising around with the level of effort that would normally have put me at about 12-14 mph, I'm more like 17-20 mph. And 24-25 mph is still fairly easy. If I want to go faster, I can switch up to level 3 and it will do 30. "All of it" will get me 35 mph. Mind you, I do have the bike geared for a wide range... I was able to hit 28 mph (downhill, pedaling my ass off) with just pedal power before putting the motor on.
Little 12Ah power tool batteries. A single one will get me something like 7 miles (depending on how I use it, of course). I carry a spare, so with the pair, I'm good for 14 miles of "assisted" travel, which is plenty for me. I went this route because the batteries are fairly cheap, easy to replace, easy to keep spares and keep them charged... and I've started collecting power tools that use the same batteries... so, that's awesome! There are TONS of battery options out there. It's all in how much range you want and how much you're willing to spend. More voltage = better power delivery, more potential torque when you need it. I'd recommend at least 48V.
Anyhow, if you're of the mind to DIY a motor kit, I highly recommend this kit from THIS seller. Fantastic service, and a great product. They'll get you everything you need and give good advice. They are very experienced with this motor and controllers, and do their own set of improvements to it to ensure that it lasts longer.
If you've been riding the same bike for 20 years, it's a quality bike, and you like it... why not just put a motor on it???
You can read about my experience here:
I ride a little for exercise and fun. I like to ride weird recumbent bikes. I like DIY projects, and I can't leave anything alone. This is just a place for me to discuss that stuff, feel free to join in.
It's Facebook, so everything is in reverse chronological order... scroll down to the old stuff first. Apparently, I've not ridden since October. I should rectify that.