While I do not know that particular model, based on my experience with diverse manufacturer's servers ...
Generally speaking, almost any (Intel-based) server that can run Linux can also run FreeBSD. But there are quite some buts, for instance manufacturer specific "add-ons", in particular firmware loaders and board/system management utilities. Some manufacturers have java based utilities which usually also work fine on FreeBSD.
Another "but" is that in most cases one can't talk about any "box X" (like "PowerEdge R420") because usually there are many configuration variants, for instance with or w/o without RAID hardware (which again comes in diverse versions/chipsets/features).
In other words: the quality of the advise we can offer is limited by the quality of your question. (Hint: Make it easy for us to answer! Needing to read through a Linux drivers list or marketing material and needing to find the relevant information ourselves, will strongly limit the inclination to help you).
So, as a reasonable minimum, tell us - in technical terms, not in the vendors marketing lingo - the exact and full specification of the box/configuration you intend to purchase. Ideally you'd also tell us about the box' intended role and the (technical) context (like: "will be used for Apache, PHP in a hosting environment with dual 100Mb links. ILO management available on board and desired").