I dont think you have to reconsider asking people to be in your wedding party, your just concerned about putting out alot of money and then something coming up and them not coming. I think there is a dif way to go about it though to protect yourself, if you're really worried, cause this goes go both ways.. it would be sh*tty of a friend to just decide last minute not to go KNOWING that you paid for all that stuff for them. If that were the case I would sure hope a true friend would offer to pay you back for all of it. So I agree with bachatchabride, If you haven't already told them about your plan to pay for alot of stuff, just have them proceed with buying it themselves (like 99% of bmaids do) and then at the wedding you can reimburse them all for it as part (or all) of their gift.
This just happened with me as I am paying for all my bmaids dresses but then my one bmaid found out she is pregnant and might not be able to come. We talked it over together and she offered, and wanted, to pay for her dress herself b/c there is a big chance she might not come. So I said that was a nice suggestion, and if she does come, I will reimburse her for it. You know? Just talk to your friends about it, communication is key.