I don't think we've ever had an unreliable Dell server, a couple have had hard-to-diagnose faults which has caused more downtime than I'd like but those are the exception.
Specifying a model comes down more to your requirements for the role it will be performing. Current;y we buy a mix of PowerEdge 1750's (1U), PowerEdge 2650's (2U), PowerEdge 2600's (5U in rack orientation) and PowerEdge 4600's (5U in rack orientation).
The 1750 and 2650 are pretty similar, we just use the 2650 when we need more PCI slots (usually for NIC's in firewalls).
The 2600/4600 we use when we want an internal DLT/LTO tape drive as these don't fit in the rack versions. Both perform very well, I can't say I've noticed the 4600 being faster than a similar spec 2600 either.
For real cheap stuff where we don't want RAID etc we get the PowerEdge 650 SCSI (you can RAID these but if the server is important enough to warrant RAID we get a 1750 instead).
As for transferring data, again a lot depends on what the server is doing. For a file server I'd build the new server alongside the old one then do a backup of the old data areas one friday night and restore them to the new server over the weekend, using something like Hyena to recreat shares. It's a hassle though if you use local groups for permissions.
For other servers we build them alongside the current ones also and transfer data if needed or if no data needs to be transferred (say a Proxy server that's not logging locally) we just swap them over.
I've never tried to do a full server restore (including configs) to a different server, you risk taking too much baggage and causing instability on the new server IMO.