Presumption is that you have at least handled bees, all the courses and all the books can not prepare the novice beekeeper for the "hands on experience" !!!
Best time is when one becomes available from your local association, most have an established breeder of local bees and will happily mentor you through those heart wrenching moments we all get in the art of beekeeping...... or a split from a colony with a laying queen of known provenance, may be as good.
BBKA advice is not to buy imported bees
Take a time served beekeeping friend with you if travelling to buy a nuc, then possibly the right questions will be asked, and don't be afraid to walk away from anything that looks iffy... difficult to do I know,...... but who would shell out £240 for a box of shaken bees and a caged queen, fresh from Bukgaria?
Overwintered bees are a good bet, I have yet to get the 120 lbs of honey from any type of bee from a nacent colony in the first year... or the second for that!
All in my own very limited experience I have to add !