I'll never have a pc with a proprietary motherboard and case. Custom is the way to go.
Every tried replacing a motherboard in a dell. You have 3 options, buy the motherboard from dell or shady online vendors: this ins not a very good option because you have to buy a board specific to dell and the model number, which has less features and cost 3 to 4 times as much. Compatible ram is another issue I have had. To get guaranteed compatible ram, it is often much more expensive than ram compatible with third party boards.
The second options which is better, but still a hassle, is to replace the motherboard and case which gets you a better board but requires purchasing a case, because third party boards do not fit dell cases. With the propietary case and motherboard gone, you basically have a custom built pc anyway, so why not just start out with one anyway.
A third option is to buy the expense 3 warranty and then extend it when it runs out, of coarse you will pay as much as you did for the pc, so it isn't viable either.
You may catch a sale and be able to buy low end pc that is just as cheap as you can build it, but if lightning or a surge, ends the life of your board, you will pay more in the long run.