100% uptime isn't something anyone can guarantee for any service. Don't make this promise. It seem you've edited your question and changed it to 99.9%. This is around 8.5 hours of downtime per year. This is much more do-able, but I would still be leery of wireless for other reasons
That being said, wireless networks are much more prone to interruption than wired connection. The per-device throughput is far lower, as well. Are you familiar with profiling RF interference? If you're going to deploy wifi as the only means of connectivity in an office, then you should be.
If you're looking to support 50 clients over wireless, if suggest you look at getting at least two 1140 series Cisco APs. These list around $800 a piece. A $40 linksys isn't going to cut it here. At that kind of money, you can grab a 24 or maybe even a 48 port SMB switch to wire in these additional computers.
I've deployed a somewhat large (400+ AP) wireless network at a mid-size university and nowhere was it the only way to connect, except outdoor areas an public areas like lobbies. All classrooms and dorm rooms had wired connections as well.
The bottom line is that it can be done, but it won't be as reliable as a wired connection and if you want it to be reliable at all, it is going to cost $ and needs to be planned by someone that understands RF interference and other 802.11 specific challenges. It's not like plugging a router in under your desk at home and calling it a day.