Sub amp options

Budget

In general, I suggest if you can't afford the options discussed here right now, wait and save. You give up a lot by saving a little, and you gain very little in spending any more.

Amp location

Do you want your amp built in? Choose a plate amp.
Do you mind another amp in your rack? In some cases, an amp can be installed out of sight.

Power requirement

This depends on your driver and the design as well as your listening habits, although keep in mind a sub will often change things for many. You might get excited and turn it up more than you expect. Simulations are a good tool for making this choice.

Typical plate amps range from 240 - 500w and they are economical in this range. Higher powered versions increase the cost a great deal and if you need more power, it's often better to use a pro amp.

Plate amp vs stand alone amp

This is the main choice. If you want a simple out of the way solution with decent power, no rack space taken up and decent power, a plate amp is the attractive option. Where more power is desired, a pro power amp is the way to go as they are the most economical option. The downside is this usually means replacing the fan with a quieter version, but instructions online are available for this. Warranty is an issue here but the procedure is quite simple. With a pro amp, an external sub pre/crossover is required. These can be purchased at a kit from places like Jaycar or Altronics in Australia, or you could also consider a MiniDSP active board.

Two recommendations

For a well rounded good value plate amp, I recommend O Audio >

The filtering options are better than most and they are generally cheaper than comparable options. Cheaper options tend to fall short.

For a good bang for buck pro amp with very high power, I recommend Behringer EP4000 >

Instructions online are available for the fan mod. This is a popular choice. You can spend more without any real benefit.

One more suggestions ...

You might also consider Rythmik audio. If you are DIY inclined, then you can buy driver and plate amp designed to go together. All of their products feature servo control, and if you aren't DIY inclined then you can buy them ready made. I have two Rythmik subs > I recommend them highly, they are very accurate subs.