From the budget point of view, ASA 5505 is an affordable box that covers a lot of basic networking needs. It has an integrated 8 port 10/100 switch with maximum 3 VLANs by default.
If you go with ASA 5510 you'll need another managed switch to do VLANs, which about doubles the cost. Sure, you can connect an unmanaged switch to 5510 interfaces, but forget about VLANs then.
It should be relatively easy to upgrade ASA 5505 to more expensive models if you need the additional scalability and features. I have seen medium-size companies (500-1000 employees) use 5505 as firewall and VPN box. On the other hand, small businesses with heavy traffic, like SaaS companies, probably need at least a 5510. Personally, I think a good scaling plan is to start with a 5505 and upgrade it to 5520 (with gigabit ethernet interfaces), if ever necessary.