As someone who started out with Dexters a few years back, to the current herd of 25, including my own bull, I would personally recommend that when starting out, you go for an older cow with calf (preferably also in calf again!). That way, you get a cow that knows what it is doing, has hopefully been well handled, you have a calf that you can get to know, and as (if) the cow is in calf, then you should be able to rely on her experience of calving to ensure that there are no problems, which is one less thing for you to worry about. My first lot were in fact a cow and her heifer calf, and an additional heifer. Having kept cattle before, I then launched myself feet first into Dexters, and promptly went out and purchased my rather lovely bull that I still have!! However, I am impetuous, and I would suggest that keeping the numbers small and using either AI or a hire-in bull would be a good move!
Catherine.