You don't need a proper drenching gun - you can get individual syringe type drenching guns, which are ideal if you have to worm small numbers. Google "drenching syringe", probably cheaper to order over the internet. a 20ml and 50ml should do most drenches (wormers and flukicides)
You will need a sprayer type thingy for applying flystrike preventative (plus obviously bottle of Crovect, or Clik - my personal preference now, or Clikzin), but hopefully now only next spring!
A halter rope - you can get them ready tied up
A can of marker spray (I have found that it is easier to have all sheep in a tight hurdle pen when treating, as easier to catch, just a wee dot of spray marker on the ones already done is necessary