alarm for 04 tacoma?

I'd use a Matrix 12.5.

How hard is it to install an alrm is an awfully loaded question. There are many factors involved, experience though is the number one factor, then getting a hold of a proper wiring diagram, any use of relays and although I doubt that vehicle will need any, diode isolation.

Get it professionally installed, so if there are ever any problems you can just take it back to the shop-but expect to pay $450-500 installed. Well worth it.

Also check out Compustar alarms.  They are a little more expensive, but they have better range and quite a few features. 

Alarms can be very difficult if you have never done it before.  Luckily the Toyota isn't too hard to do as most of the wires can be found at the fuse box.  You will have to diode isolate the door triggers, and to test for lock/unlock you have to use the key in the door cylindar.  Beyond that the install should be pretty simple.  Just keep in mind that the harder it is for a thief to find the alarm or it's wiring, the more secure the vehicle will be. 

The number one thing you want to do is test every wire to be sure you are using the right wire.  Don't just assume you have the correct vehicle wire as the vehicle wiring information can be wrong.

The second thing is to hide everything, including your wires.  You want to make it where if a thief looked under the dash, they couldn't find any of your wires or the brain.  This will prevent them from being able to circumvent the alarm system.  The most expensive alarm systems are useless if they are easily and quickly disabled due to poor installation.

I would also suggest bench-preping the alarm.  This will save you some time and clean up the install.  What you want to do is cut off any wires you aren't going to be using, and combine any wires that will be going to similar places in the vehicle into their own harnesses. 

And finally I recommend you reading the information available on the left hand bar on this site.  If you run into any problems or don't understand something, a quick search on this site should answer your question.

Good luck!