Hi,
Hope you don't mind me adding my 2p's worth.
I would recommend mixing by weight straight of the bat. I have been diy-ing for a long time and I can honestly say whatever the costs for scales it is worth it. Mixing testers for juices is something you will need to do to avoid waste and with scales you can easily mix straight into a 10ml bottle, this will save you so much time from washing and drying gear and personally I find it more accurate which is great when you want to upscale from a 10ml tester to a 250ml mix.
I would recommend some kind of milk frother for mixing juices over 50ml, the argos ones look identical to the asda ones but are better.
I mix large amounts in glass ware, a 250ml flask mainly. Storage was mainly in glass but I have moved to plastic recently as it is cheaper and no one ever gives me the bottles back.
Most flavour suppliers will provide you with pg/vg in a litre bottle for about a tenner I would go to Lucemill as its so much cheaper to buy 2.5 to 5lt. Decant some of this into a couple of bottles you can drip from, say about 100ml, then you can easily pour in the bulk of what you need from a smaller bottle and then 'drip' the last few ml/mg in to be accurate.
Vapable are good for concentrates and have a wide selection also Chefs vapour as others have said. Their Welsh Custard is worth a look if you are into custards.