One thing that I've learned is that buying a guitar isn't like buying a keyboard. You can't just look at it online and expect it to be good. You have to touch it, play it, and examine it. If you buy a cheap one, the frets could stick out too far, slicing your fingers as you play. However, since I know that you've been playing boards for a while and seem to be pretty good, I know that you should have a pretty good ear. This is really important when learning a new instrument. All you'll really have to do to start playing right away is learn to finger a bit and practice your progressions. This is all you really need to know to play at an ok level. It sounds bad trying to play extra stuff badly, as a beginner, than it does when you play straightforward with the regular notes. You'll learn all that stuff later.
For me, learning to finger was harder than learning to slap, so I'm actually more acurate slapping than fingering--people tell me this is backwards. I recommend though, that you could get a good 500-800 bass and use that to practice with then once you learn your fingering and progressions, you won't feel the need to get a new one because it will have some quality of sound in it. Test out active and passive basses.
As far as bas amps, you can get any one that's in the 150-300 range and it'll be good enough to practice and use in a small church. The one that I have is a Peavey Basic 112, which I bought from a friend of mine for $150, a steal, since it costs about $250 (I think) online.
