I wonder in particular what the average Sami would have looked like during the 5th century. Today Sami people are fairly diverse in appearance, some bearing Finno-Ugrian traits like high cheekbones and hooded eyes, and others looking indistinguishable from people of Norse descent. Were the Sami that diverse in the 5th century, or would they have looked more like the other native Arctic peoples?
Any resources you might recommend about Sami culture in general would also be greatly appreciated.
Very little. The first possible reference to Sami (or rather, to Finns) occurs in Tacitus' Germania which in chapter 46 describes them as a hardy, rugged, horseless nomadic people, a sort of "noble savages" who hunt with arrows of bone, since iron is so scarce. In the 6th and 7th century we see e.g. the goth Jordanes refer to "scerefennae", often taken to mean "skiing finns", supposedly the only "wild", non-settled people in Scandinavia. These have traditionally been identified with Sami.
The first detailed, unambiguous description is given by the 9th century wealthy Norwegian landowner Othere in the Anglo-Saxon Chronicles, who boasts of his vast properties and the fine animal skins that Sami pay him in tribute. Particularly he mentions owning 600 "unbought" reindeer, and the use of reindeer as hunting decoys, which makes the account significant and somewhat contentious. Some have held it as evidence for large-scale reindeer husbandry by Sami while others maintain Sami only engaged in that starting in the Early Modern era.
Unfortunately, much of the evidence available is more anthropo- and archaeological, as well as linguistic, than historical; many of the earliest historical accounts are rather stereotyped, written by people who had most likely never met a Finn or Sami.