If I was starting again I'd buy things as I went along as it's much cheaper that way and you don't end up with loads of things you just might use one day but far off in the future when you can't find it. The stock/parts/materials I hold might be very different from your requirements as the customers I have around here are probably different from the customers who call on your services.
Slightly more positively in answering your question, I've just returned from holiday and had to purchase a load of things as I'd run out of most of my common items on my last job.
20m copper tube (15mm)
End feed 15mm ... 100 x elbow, 25 x street elbow, 25 x equal tee, 25 x coupler, 25 x street elbow, 25 x stop end
Compression 15mm ... 2 x equal tee, 2 x elbow
Outside tap x 3
Screws: 200 x 4.0x40mm, 200 x 4.0x50mm, 100 x 5.0x60mm, 100 x 5.0x70mm
End feed 22mm ... 20 x elbow, 20 x coupler, 25 x stop end, 10 x 22/22/15 tee, 10 x 22/15/22 tee
20 rolls of pfte tape, flux, solder, heat mat
I've got stocks of other items but tend only to keep a length or two of 22mm pipe, some oil pipe and 10mm pipe but no plastic pipe although I keep fittings. I also have a loo syphon, a few inlet valves, a part 2 valve and 3 organisers full of various odd fittings like tap connectors but these are left overs from other jobs or collected along the way thinking they might come in useful one day. I've still two 22mm gate valves I bought over three years ago. I've still a basin wrench I never used. I've still two enormous stillsons I've never used. I've still ... etc, etc.
I've 3-6 outside taps in stock but I've only fitted these in rural properties and I live in a rural area. I think I've only fitted one in a town. Likewise I've hardly ever touched oil within a 30mph limit so the list above might not be suitable for you to copy like for like, but I hope it helps.
I spent a good 2-3 hours with a Screwfix catalogue when I started as they have most of the common parts you'll need in an easily laid out catalogue. Occasionally I have a good read through (then see if I can find the stuff cheaper else where!)