1. The number of nodes is not fixed.
2. The process for node selection is random, based on the amount staked. This happens continuously.
3. The D in DPoS is entirely optional. Anyone holding Tezos can directly participate by running a node. The only cost is the cost of a VPS.
The only real restriction* is a 10000 XTZ minimum for running a node. At current / ICO prices that's around $5k. While that's not cheap it's a lot more attractive than the CAPEX/OPEX of mining hardware.
Also, that $5k will grow by 5.5%++ per year, i.e., no depreciation of hardware and only a trivial amount of overhead.
Those not wanting to run a node (or with less than 10k XTZ) can delegate to a baker. There are a wealth of them available already.
*This restriction may be lowered in the future -- by way of on-chain voting (as opposed to a hard fork).