Instant weapon change

Go to Options, open the Keyboard tab, click Advanced... and then tick the "Fast weapon switch" option. This'll make switching instant when you press a number key or scroll.

In addition to the fast weapon switch previously mentioned, what I do is rebind:

Primary weapon to mousewheel-up

Pistol to mousewheel-down

Melee weapon to mousewheel-click

That way, I never scroll past a weapon I want. All you have to do is flick the mousewheel.

I have a 5 button mouse. So I use bind my spare buttons directly to my primary and alternate weapons. It doesn't work that great for spy/engineer, but for a medic where I am mostly using the Medigun/Syringe gun it is great.

I have updated my autoexec.cfg like so.

bind "MOUSE3" "slot3; +attack; wait 50; -attack"
bind "MOUSE4" "slot2; +attack; wait 50; -attack"
bind "MOUSE5" "slot1; +attack; wait 50; -attack"