Not all cycle insurance policies have an excess and it can be low on a home contents policy too.
Why do you absolutely need slick tyres, plenty of people ride happily on tyres with tread?
If buy second hand from a bike shop or charity, it will have been serviced
You need reliable , puncture protected tyres that are not rotted with age or worn out. Tyres that will get you to work on time, every day. If you are outfitting an old bike for commuting, you almost certainly need new tyres and they may as well be suited to your commute.
Slicks can be taken to mean "not knobbly", rather than totally treadless. Bog std Marathon is ideal for a reliable, short urban commute