Which car features are more cost effective if purchased & installed aftermarket, as opposed to when buying the car?

Keyless entry and stereo are the only items that come to mind on that list as having after-market potential. If you buy a used car, seat could be an option, but you would probably give up power controls and things like heating unless you are a real car guy.

In general, if you're buying a new car, I think it makes sense to negotiate as much as possible during the sale. Accessories are always high margin, and you have leverage when you're buying the car.

If you want fancy aftermarket stuff, you probably want to buy a nice, clean used car and fund your aftermarket stuff with the savings!

Most of the new cars should come with the following in the basic model:

1. Remote Keyless Entry
2. Stereo with CD
3. Cruise Control
4. Power Windows/Locks

The rest of the features will be extra and usually a part of a "package" (like Bluetooth comes with the "connectivity package") that is offered when you buy the higher trim levels. For example the first trim level above basic might come with a nicer stereo with sound system, but you might have to buy the 3rd or top trim level in order to get leather seats and/or sunroof.

Typically you can pay extra to get the Bluetooth with most of the trim levels, or have it installed for cheaper after you guy the vehicle. (One note if you are buying a Chevy, if you don't buy the factory Bluetooth package and want to put in an aftermarket one you have to disable the Onstar system in order to be able to run the Bluetooth though the car sound system)

Automatic Transmission is typically $500-$1000 more expensive than getting a manual, although many vehicles do not offer a manual option these days.

GPS can be a money saver to add as an aftermarket item as opposed to getting an integrated version with your stereo. Typically, updates for the aftermarket stand-alone GPS units are cheaper as well.

Good question! FWIW, I was given a price of about $600 for aftermarket cruise control on a Hyundai Elantra. One problem with trying to get some-but-not-all of the features is that you can't pick and choose -- you have to get the features as part of a package. Cruise was the only thing I really wanted (i.e. would have been willing to pay extra for) from the next-higher trimline, so I decided to drop it. The particular set of features you're looking for means you're going to have to buy some stuff you maybe don't want (alloy wheels or something like that).