You probably know this bit: 700c is a seemingly meaningless number that refers to the size of the wheel, a bit like how a 24" or 27" wheel is not actually 24" or 27". 20c is the width of the tyre in millemetres, so 20mm. The old size 27 x 1-1/4" tyre is about 32mm wide, for reference.
20mm is pretty narrow and the common size for fast road tyres is 23mm. There are a lot more 700x23c around then there are 700x20c. I find 23's are ok if the road surface is good, but add some pot-holes, cracks and general detritus and they can be a bit of a rough ride and a 25mm tyre is a lot more comfortable. You can get most tyres in both, so basically take your pick. £10-£15 buys you a good quality budget tyre, £25 buys you a lightweight high performance tyre. Continental Gatorskins are regarded as tough tyres with lots of puncture resistance, as are Vittoria Rubino (my choice) and Schwalbe Ultra Marathons. Schwalbe Ultremo, Continental GP4 and Michelin Pro are more at the performance end - great tyres but a bit prone to punctures if you commute.
Brake blocks - this is only my personal experience but it depends on the rim and brake caliper. If you have older Weinmann calipers then they work better with old type brake blocks, so look for rubber ones. Either the old shiny rectangular ones, or you can get a more modern looking short oval shape resin one if you have a 90's bike. If you have newer dual pivot calipers and new hard anodised alloy rims then use those long skinny cartridge brake blocks where you can replace the lining. If you're like me and use a combination of the above parts, then you might need to experiment a bit.
Lube is your choice basically. I can't see any value in using anything more than run-of-the-mill white lithium grease in bottom brackets, wheel bearings and headsets. For chains and gears, don't use WD40 because it's mainly solvent. Use a good quality bike oil and you can get different ones designed for wet or dry weather if you don't want to use a universal one. I got the 3-in-1 spray bicycle lube from Wilkinsons but I don't like it. Plus it sprays everywhere, so look for something in a normal oil bottle