If your site is important enough that you want it online 100% go for a load-balancing setup. If it's not important enough to spring for load-balancing then chances are you'll survive a little downtime.

I'm not trying to be rude here, but you're not going to get what you need for "cheap" or "free" so if your budget isn't reasonable then you will have to find some way to handle downtime that is less expensive but then again not such a great option overall such as manual/automatic DNS failover and manual/automatic data replication usually via CRON.