Another school of thought - the less you spend now, the less you lose. Every extra $£ you spend on getting a better model will have to go at a discount because you are selling a used machine.
E.g if you spent $xxx on upgrading the HD to the max available now, it will be worth a lot less when you come to sell if aftermarket upgrades become available and capacities/speeds increase.
There are just too many variables and unknowns to give you a decent prediction right now. Just buy the machine you want and need right now and worry about resale value if and when it happens.
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people who buy used are generally not very knowledgeable or very knowledgeable.
The very knowledgeable one will pick up a good used mac that had apple care for a good price, I check 10 times a day for used stuff. When a deal comes up I'm picking it up within an hour.
For the not so knowledgeable buyer they want to get into the mac market as cheap as possible and will give you almost no price premiums for upgraded apple products.
Buy what you need, nothing more.
Another great way to get more for your apple stuff is to add adapters and hdmi cable that you can for cheap from china. Add a Mini DP to HDMI adapter, HDMI cable, cheap case and other stuff. Since they are not so knowledgeable they may pay a premium for the extra stuff.