There are actually also some "pre-made" registrars out for sale in the market. Basicallly it works by handing over control over the company that owns the ICANN accreditation.
Biggest challenges I've seen so far where:
* Getting the liability insurance, which can be quite expensive, especially if your company is locate in a tax beneficial country.
Some of your running cost (might be a little out of date):
$4,000 ANNUAL ICANN LICENSE FEE
$8,000 ANNUAL VARIABLE REGISTRAR LICENSE FEE (BASED ON CURRENT YEAR FEES, FOR LOW-VOLUME REGISTRARS)
$8,000 INSURANCE (ESTIMATED)
You also need to proof that your company has got at least $70,000 of working capital