Electric Bikes - Need advice on picking an electric bike

Hi I'm new to electric bikes and want to try them out.
I would like to find a bike that < $800 used.

Some backstory. I will be commuting anywhere between 6-10 miles one way next month. I'd like to use an electric bike to do it. The route is not too hilly, but is mostly on roads in the business area of town.

Some requirements for the bike:
1. Be able to maintain 20-25mph with easy pedaling
2. Can survive my full day's commute (12-20 miles) in one charge.
3. Can maintain at least 15 mph without battery power (so not too heavy of a bike).

Some background on me. 2 years ago I used a road bike to commute 8-10 miles every day (holding a speed of 18-20mph). I'm not in shape now, but I can get back into that shape. I'm in my 20's.

Just some questions:
1. Can I ride an ebike in the rain without damage?
2. Are there speed limits to these bikes?

Basically I've searched on Craigslist and found lots of electric bikes. But I'm not familiar with them so i dont know how to tell what is good or not. Like some have motor in the front wheel vs back wheel. Some have a motor that just rubs on the rear wheel, or that drives the front chain ring. Or some have frames specially built to hold the battery (ebike), while some (like ezip) have the battery on a rack.

Which are the older or inferior models? Which is better for my needs?
What should I look out for when looking at these bikes?

Could someone summarize like a few majors brands and models of electric bikes with pros and cons? Like why would 1 choose an ezip or ebike?

Should I look for bikes built looking like scooters? Or bikes that accomodate the battery and motor? Or bikes that look normal but just have some electric motor attachment?

And is a DIY ebike better? I have an old Scott Summit that can be a donor bike.

Don't bother with any that have something that rubs the wheel, it just wears out the wheel and is an old idea that isn't needed anymore.
Personally I like bicycles that have been converted to electric because I can replace the parts if they break with other parts. Many of the made as ebikes have special parts that won't be available when they go out of business in 5 years or stop making that model. If you had a lot of hills or were carrying cargo then I think that inline systems are the best. For commuting a rear hub -front hub can't be as powerful as it could break the fork- might be the easiest for you.
To increase distance just increase amp/hour in the battery size you get. It is easy to figure 1 amp/hour per mile. You will probably have more than you need but as the battery wears down you will still have enough. I suggest to newbies to electric that they start with lead acid batteries and when they wear out in 6 months or a year they can buy lithium batteries which are more expensive up front but are cheaper if you are sure you are going to use them because they last so much longer. If you don't like electric, you can sell the kit but you can't get hardly any money for the batteries because no one can tell how much life they have left in them which is why I suggest you start with cheap lead acid.
The scooter like things aren't bicycles so you won't be able to pedal them in case of a failure in the electric system.

Thanks for the advice crackerdog.
Ya I guess i'll be looking to mountain/hybrid bikes converted to electric then.

A rear hub motor sounds good but that's seems more complicated because all the gears are there.