Where I work, there are showers and a changing room which I used to use. But one day I realised that if I cycled slower so I didn’t sweat, my door-to-desk was faster because I didn’t have to waste time showering and changing. So I dress for work and cycle at a relaxed pace. I’m a big believer in wearing ‘normal’ clothes and hopping on and off my bike.
As for tyres, I like a ‘touring’ tyre – 1.5" wide with some grip, but no knobbly bits. This is pretty fast but can also deal with the weather, rough roads, grit, gravel paths, etc. Something like the Schwalbe Marathon.
If there’s a bike you like, but don’t think the tyres are what you want (too thick/thin; too much/little tread), talk to the Evans staff. They will likely be willing to replace the tyres for you and add the cost onto your Cycle to Work voucher, so it gets paid for all as one. (Note you can include clothing, helmets, locks, etc. on the voucher too.) Buying through the Cycle to Work scheme is definitely the way to go – as well as spreading out the payments, it saves a lot of money.
You can measure this in decades if you replace parts when they break or wear out. Unless the frame gets damaged, anything else can be replaced (and the frame should last well over 50 years). I bought my bike second-hand, but over many years have had to replace most of the parts at one time or another. But it keeps going, it’s a workhorse. It really depends what you want out of a bike. Some people want something a bit fancier or to be able to take advantage of new gear as it comes along. If that’s important, at a certain point it makes more economic sense to sell your bike and buy a new one.