Ibanez has been mentioned already, but they have a good selection of five strings at affordable prices. The Soundgear series comes to mind, although I think the best bang for your buck is the SR505- solid, great price, Bartolini pickups... just a great bass! Even the SX basses on
www.rondomusic.com are a great deal.... $200.00 or less...
I would say the five string has been a relatively recent addition to the bass world, and lots of gospel music was played on a four string. Don't get pressured into buying a bass you may only need to use on a couple of songs; and I would say that is the key - that the instrument needed for a song depends on what range the song/instrument was intended for. I say let that be your guide... if most of the songs you're asked to play favor a five string, because the song has notes played below "E", then you will have to strongly consider the "B" string...
It sounds like the people asking for a five string want you to play the low organ notes. If there is no organ in the band, I can see why they want you to have a five string. If there is an organ in the band, then I don't think you should have to play in the same range as the organ.
One last thing - as far as flat keys go - you can always tune down a half-step to "Eb", or drop down to a whole step to "D"... these are options you can try before having to spend more money.... or, you can ask the people who think you need to buy a five string bass to contribute to your purchase, if they think it's so necessary!