That is a tall order to fill, you want a quickly removed electric drive train that costs $300 or less, and can go 25-30mph... wow, good luck!
For about $300 you can get a
cheap toy kids electric bike with a low quality motor, normaly already on a cheap kids bike, but you could swap the parts over to the bike of your choice, lots of people do it! they should give you a top speed of 10-15mph (17mph with a small child), and claimed range of 10 miles, but range is a governed by the number of batteries.
$450 you can get a
ZAP bike kit, but that will only give you a 15mph top speed and an 10-15 mile range depending on hills, of course you can add a 2nd battery and double your range, but not your top speed, the big advantage of this kit is that you can easily remove the battery, then completely disconnect the electric motor from the wheel, altho it's still attached to the bike, of course this is a disadvantage as well because it's an inefficient drive train to have friction drive like this, but it only leaves about 6 pounds of extra weight on your bike when you want a "normal" bike.
$450-550 can get you a
Sparrow it has a 22-26mph top speed (depending on if you get a 36V or 48V kit) and about a 10-13 mile range, but you can add more batteries, these kits are the lowest quality that I would ever recommend to someone, they have a hub motor, that means fewer parts and less wear, of course to remove one of these kits you would have to swap wheels, remove wiring, remove the batteries, and if you want a nice clean looking e-bike, it takes time to get the wiring and controls attached to firmly to the frame.