Ian,
That's a question which could extract a very long reply. Kits - simply you generally pay for what you get, the cheaper the kit the poorer the engineering. My favoured supplier is Woodturningz.com, timberits.com (as I order in volumes in excess of $200 I get them TNT free and they turn up in a few days - I ordereed last Thursday and I know they are at the TNT depot in Luton and will be with me tomorrow. UK suppliers, sorry Axminster but for all the great levels of service and other kit you supply the pens are awful (but getting better slowly) There are problems with the engineering on some of the more established kits because the kits are metoo kits from Chinese suppliers (the sierra from Ax is the same as the Mesa from woodturningz and the twist mechanism is too small resulting in the twisting of the top to reveal the nib to turen without pushing the nib out.) Too much detail!
Refills - most take Parkers, slimlines and variants take the cross, rollerballs take a variety of refills some are awful, the best are schmidt and it is wise to replace the C**p ones with a good one from Schmidt if you are aiming to sell, its a much better writing experience. You don't have to turn to bushings but it does make life easier. Welcome to a whole minefield, its a fun learning experience. If anyones visiting Lincoln Christmas Mmarket this coming weekend, look me up I'm stand 309 in the Christmas Bazaar Marquee, trading as Crafted Pens!
One piece of excellent advice I took from George Watkins (Cornucopia) - when I started out he told me lifes too short to turn rubbish wood. Dead right George, particularly on a pen, if its not top notch wood its a waste of a kit!