Should I upgrade my mid-2012 MacBook Pro?

Your quoted price of a new top case replacement is close to the price of a used mid-2012. I like mine but I’m realistic about what it’s worth in resale. I don’t think you would get much value in replacing it unless perhaps you’re willing to do it yourself with salvage parts. It’s not easy, but there are step by step guides that can help.

Obviously you’re willing to use it with an external keyboard, but of course a buyer may consider that an obstacle.

If you hope to sell a working computer then you need to replace the keyboard and battery. Without those, it is not worth much for trade-in. Your model is still supported which means you cannot buy parts from Apple. You will have to look on eBay or iFixIt.com for a keyboard that you can replace yourself. You can buy a good quality battery at OWC.

If you don't wish to put more money into it then consider a new machine or a refurbished one. Refurbs from Apple are a good choice since they come with Apple's one-year standard warranty plus they are eligible for AppleCare.

If you feel it's worth it to you, it's worth it. The model you have is one of the most reliable ones. A new keyboard and it should last a couple more years. It's also easily upgradable with a new SSD should you fancy. Also, should you decide to replace the battery later on, it's so easy anyone can do it. (When purchasing, the number of the battery is A1322.)