I think I’d definitely want a second pair of hands - things like cabinet hanging are easier with two and it’s easier to sort the plumbing if there’s someone to hand you things.
I have a galley kitchen and just had a single length of worktop. I used solid wood, and cutting the sink hole was pretty challenging. The good thing about wood is any abutting pieces are easier. I was also amazed at the price: it requires maintenance and you can’t exactly scrub it but it was vastly the most economical way of making the kitchen look really smart.
With cabinets, I built them my self - just bog standard b and q frames, with decent hinges and fully sealed doors (to prevent warping). Cabinets come in at least 15, 30, 40, 50 and 60cm widths, so with a bit of wiggle room around dishwasher and washing machine you can generally avoid fillers.
They sell a handy socket converter so you can rewire a two plug socket to 3, which can be handy for electrics like if you want to add a dishwasher.
Second hand appliances (and even while kitchens!) are often on gum tree. I saw a set of neff appliances on the customer board in Tesco for £150 the lot the other day!
If it’s going to involve a layout change remember to cost in flooring: my kitchen had nothing underneath the cabinets.
Make sure also that you leave enough time: it took us about 3-4 days to fit.
Don’t reuse pipe and don’t reuse wire. It always ends in tears and isn’t worth the saving. Buy good quality braided tailpipes for the taps as if they split it will flood your house. Hinges and even cabinets can be salvaged if they haven’t warped - sometimes taking the cabinets down, giving them a good wash and replacing the door can be enough of a freshener to save the kitchen.