Upgrading worn drivetrain

[Seek]

I've just been gifted a well-used touring/city bike by a friendly neighbor.

My intention is to use this as a local beater for quick shop/pub trips as well as some basic touring. However, it could also possibly be a back-up for winter training.

Quick inspection reveals:
- Shimano Altus drivetrain
- 3-speed crankset (small cog worn)
- 7-speed cassette (seems ok-ish)
- rear derailleur - cogs almost worn down to circles

Any resident techies want to provide advice on the route to go?
- replace 2 rear derailleur cogs + chain. Ignore the rest
- option1 + upgrade crankset + cassette with straight swap 9-speed
- replace everything
- toss the bike and buy something else

[WinstonWolf]

2 rear cogs are a fiver, if the chain is still serviceable just replace the cogs and rid it. Pub bikes are supposed to be cheap

[SuperHangOn]

Cogs + £4 asda chain if you're feeling posh. Clunkernomics.

[gazza285]

They are known as jockey wheels and are sprockets, not cogs. Cheap to replace.

If the wear on the rest of the drivetrain is similar, then you would be better off either replacing the whole lot, or keeping what you have, old and new don't go together well.