Well, Dell isn't a bad choice. They have their foibles though. Proprietary power supplies, 5-pin fans, complex power/reset switches. There are adapters for the power supply, you can rig the computer to turn on when power is applied, and re-wiring the fans is pretty quick. Dell boards from Micro Towers are usually ATX compatible, so you can swap to a bigger chassis to mount a proper power supply.
Lenovo and HP are fairly popular conversions as well.
ASUS pre-builts are usually pretty much straight standard computers, since ASUS makes their own boards they don't bother with proprietary stuff.
Not sure about Acer.
Optiplex 9020 are about the sweet spot in terms of cost. Fairly easy to get an i7-4770 or i7-4790 for around $250. That will usually include 8GB of memory, at least a 500GB hard drive, and Windows. If you shop around you can often find SSDs pre-installed.
See Moddiy.com (many models) for power supply adapters if you don't want to make your own. You can find them on Amazon and Ebay as well. Just search for Make, #pins, and power supply adapter, usually the right thing pops up.