Being an OEM motherboard, you'll have to check what CPUs they officially support, and of course make sure you have the latest BIOS installed before doing anything. The simplest option would be the 1400, as it's low power and definitely will be faster than the A10, you already have a dedicated GPU as well so that's not an issue.
When you look at higher performance models however, you'll want to be sure of two things;
- the TDP supported by the board (a lot of OEM's are only 65W), which shouldn't be a huge issue, but a high TDP model might need a fan over the VRMs.
- the RAM, ryzen has a lot of performance, but if you only have single-channel memory and at low clocks with loose timings, that performance won't be usable as it'll be constantly waiting for the RAM to complete data transactions.
There's another possible issue though that the board could rely on OEM-specific hardware, which means that standard off-the-shelf parts may not work in the system, and the existing parts may not work in an off-the-shelf motherboard either, so I'd be contacting DELL and trying to get as much info out of them as possible...