Welcome to the forums.
For your question, when you say shows signs of wear and tear, is this cosmetic, or physical? a cassette can look worn after a few rides, but not have any real wear for thousands of miles use. What is the chain doing when riding? if it's jumping, the cassette is probably worn, if not, would leave it until this starts.
If the cassette is worn out, you will also need to replace the chain (would replace with any change of the cassette), would be looking at the condition of the chain rings (on the crank) and the pulley / jockey wheels, as after 3-4 years, all of these may be worn out depending on how much the bike has been used.
For the bike shops attitude, if you are a 'noobie' swapping a cassette is a pretty specific thing to do, and indicates some knowledge of bikes, so maybe they think you know more than you do, perhaps ask them more general questions rather than being specific to give them an idea of your level of knowledge of bikes.
For your choices, why not just stick with an 8 speed (changing to 9/10/11 speed will be very expensive) and stick with the same ratios as you originally had. What about a SRAM PG-850 12-26! & a new chain.