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#1 Best way, if supported on your IOS, is OER/PfR for outbound.
Inbound is a problem. With your two public class Cs, you could advertise one as "better" on the 8 & 16 Mbps links, and the other as "better" on the 24 Mbps link. You could also talk to you providers about what they are willing to do, such as passing the preferences in community strings. All inbound, though, is going to be impacted by what's further downstream so it's not 100% unless instead of advertising one class C subnet as "better" then the other as described, you only advertise the particular subnet and only provide the other only if its link fails. Even doing all this assumes your two public class C subnets will draw equal traffic.
#2 The only major item I notice, you have 8 Mbps one router and the 16 & 24 Mbps on the other. If the latter fails it takes most of your bandwidth. Better would be 8 & 16 Mbps on one router with the 24 Mbps on the other. Doing this guarantees you won't lose more than half your bandwidth and also likely better distributes the load across the two routers.
#3 Normally really required if your taking more than a default route from your providers so all your internal routers know the external routes. Taking just an external you don't really need to run iBGP, but you can still pass the default given to you from your providers to the other iBGP router. For example, right now if your 8 Mbps link failed, but your router doesn't, how does that router know to use your other router? Probably via iBGP.
PS:
One issue, which is difficult to deal with without OER/PfR, how do you direct traffic to your two internal BGP routers? By default, equal AS paths will use the egress router you're on, so, for instance, if traffic starts on the 8 Mbps router it won't use the 16/24 Mbps (this might be why you're configured with 8 Mbps on the one router, as a backup, you might direct all traffic to the 16/24 - and even then, it will prefer one link, and it might not be the 24).
