Shouldnt be a problem mate, ive read that brass can go all gungy after a while but as long as you are particular with your cleaning you should get a bit of life out of it, id say being on a fermenter will be even better than being on a kettle cos its not going to be exposed to extreme heat...
Id say go for it, and start saving your pennys for a ss ball valve as an upgrade at a later date?
Stainless is not necessarily more expensive. It depends where you get it from. I needed a 1/2" BSP cross for my fermenter and tried the local hydraulic hose company. They quoted me $52 for a brass one. I rang Ray at Anzor Fasteners in Wellington and bought a stainless one for under $12!. Anzor also have stainless ball valves which are probably cheaper than you can get a zinc plated brass one for from you local plumbing merchant. Shop around.
I have no connection with Anzor other than as a very satisfied customer.
I'm going to ring in here to, with no affliation, but Anzor are awesome. In fact I was quoted less for a stainless elbow, than I was for a brass one from another supplier.... nothing makes you feel like a valued customer when you hear the words "we'll give you a good price", and you get it!
It's funny how when you ask for a cash price you get $36+GST, then you say screw that and what's the price on "x" account and it's $19+GST. 50% off when using a trade account, now that's what it's about nowadays.