I started with Dexters many years ago and still have one cow descended from the original one. They are a good breed to start with but tend to have small person syndrome, and can be difficult if not well handled.
I since went on to Herefords and they are far more placid by nature.
Are you intending to eat the meat that you produce yourself, or will you sell it at market? If the former, then Dexters are good choice as you won't end up with a mountain of meat that you have to deal with. Half a Dexter will easily fit in a family freezer. A Hereford produces considerably more. If you plan on selling the animals fat then you would be better with Herefords as they should fetch a decent price in market, whereas pure Dexters do not because of the small size of their carcass.
The Herefords bought by your neighbour are likely to be dairy bred crosses which may not have as good a conformation as beef bred animals. (like comparing the difference between a chunky labrador with a labrador crossed with a greyhound.) But at the right price they could be ok. So how well do you trust the judgement and honesty of your neighbour? If you're happy to deal with him, then I'd go with his choice.
Obviously you don't want to be buying in diseased animals but to get any with a vet's certificate is going to cost you more and is difficult to justify. It is normally only done where you keep exclusive pedigree animals where a breakdown in heath of the herd would cost you thousands in lost sales. I, in 30 years of selling privately have only once been asked for BVD and Johnnes testing and that was for some highlands I sent up to the Shetland Isles which is a high health status area. (Yes they passed!)