Unimat mill is even smaller than my Sherline, so not sure what size vise to recommend, but I have a 2" screwless vice which has worked well for me. Might fit on a Unimat, or you might want to go down a size, maybe 1-1/2".
My Sherline has a 2.75x12" table , so you can compare to the table on yours to help decide on a good size for you.
You can find vises of this type on ebay usually for $30-60. Many have round holes, I had to look around to find one with a slot which works better with Sherline's hold downs.
Collet blocks are handy as well, ER32 and 5C are the most common but you might be able to find ER16 to match your collet set.
Both can be seen in this photo on my mill.
I've been satisfied with the end mill sets I've gotten from Little Machine Shop. I've used up to 3/8" end mills on my little mill, so the suggestion to just order a couple different 1/4" end mills, maybe also some 1/8" is probably a pretty good idea. Most sets go up to 1/2" so sets are probably not that useful for the Unimat.
I wouldn't go to nuts on different cutters until you actually have a need.
Slitting saw arbor and some different size slitting saws may be useful.
You are going to want a small hammer and a set of parallels to use with the vise.
There are a couple of books specifically for the Unimat. I've got one called The Book of the Unimat by DJ Laidlaw Dickson which is a pretty good overview of the different models, what to expect from them, accessories, modifications etc.
There is another called Making the most of the Unimat by Rex Tingey. I don't have this one, but appears to be similar in intent to the Book of the Unimat. There was a companion book Projects for the Unimat also by Tingey.
Both of these were written in the 1970s and out of print but not too hard to find used. I've found much better prices at Abe Books where you are mostly dealing with used book sellers, than Ebay or Amazon where the sellers include terms like rare... England seems to have a more active small machining community (model engineering as they call it). You find a lot more copies being sold in the UK so keep an eye on where the seller is or shipping may be more than you expected.