Ugh, I walked 7 miles yesterday in the driving sleet. It is nearly the end of April FFS. I have courgettes and squashes in the living room, while the kitchen is full of tomatoes and aubergines and chillis and peppers. It's absolutely mad. This so-called 'spring' is getting me down.
notsaying - the mix of grit/sand and soil depends a bit on what you are growing and a bit on the state of the soil - the claggier bits of your plot will probably need more grit than the soiley bits! In my asparagus bed, which is about 3 metres by 2, there are around 7 whole bags of horticultural grit. I'm also going to grow sea kale, which will need similar quantities though in a much smaller area. Elsewhere, though, I've used far less than that and none around raspberries and rhubarb.
I appreciate that this isn't really helpful for your ordering process! I think, given that this is an allotment and not a back garden and there is clearly a drainage issue, I'd be tempted to invest in a big quantity of horticultural grit (£60-80 or so) and then to try to use free manure, free spent hops or mushroom compost for now. Then I'd add in cheap compost/more manure when it came to planting up, depending on what I was growing (many roots, for example, don't like too much manure, but will enjoy compost; courgettes need lots of food so loads of manure). Bear in mind that, apart from the waterlogged areas where it probably all needs digging over, you only really need to improve soil in bed areas - paths don't need rich, lovely, expensive stuff on them!
If you use something like claybreaker (gypsum) that does need to be measured out so you don't end up changing the soil too much. Instructions about quantities on the packet.
You can also use your crops to help you out - putting potatoes in a bed really helps to turn the soil.