Not enough information to accurately answer your questions, but for some "food for thought", a L3 switch might suffice, alone, at your new site if it supports the necessary physical handoffs (e.g. Ethernet copper/fiber) and supports your routing methodology (e.g. RIP/OSPF/BGP).
Using an "ordinary" LAN L3 switch, in this type of situation is often the most inexpensive option, but not always the best. Such switches are designed to provide maximum performance, often at the loss of many of the features found on most WAN routers. There are a class of L3 switches, that add back some of the more important WAN features, often known as Metro Ethernet switches. An example of a comparison of these two types of L3 switches is Cisco's 3750 Series vs. their 3750 Metro Series.
Aside from using either type of switch, you might want to carefully consider whether a router would be better. The current ISR routers, besides having almost any WAN feature you might need, offer a extensive set of options. For example, in your situation if you used a 3845, it could support Ethernet switch module(s) (provide the Ethernet ports for local hosts) and/or WAAS (Wide Area Application Services - attempt to provide WAN access at LAN performance) module within the router chassis.
With regard to you questions about MPLS, MPLS usually ties together different sites, often such they all can directly access all the others. There are several different ways of doing this with MPLS. It would be unusual to use two providers for different sites, since different MPLS providers often do not communicate with each other. Perhaps what you have in mind is different local ISPs providing the link from each of your sites to the same MPLS provider.
There are several methods to have automatic failover between you primary link and the secondary. It also might be possible to use both at the same time. How either is accomplished is dependent on your equipment capabilities and how its been configured.
