changing coolant would be a good idea, yes - and check the hoses while you are at it. it is not 100% necessary to drain/change it, timing can be done without draining... just pull the fans for better access, but if you are doing the job, do it all and be good to go for many miles.
Aisin is the OE water pump supplier, and they do offer timing kits, but you need to know which motor you have.
The absolute MOST important part to change, besides the belt, is the cogged idler pulley, which tends to be the one that fails first out of all of them. Cam & crank oil seals are definitely a good idea.. Tensioner, unless it is leaking oil, is not a really a necessity, but good practice if you are unsure.
the plate on the left (drivers side) strut tower will tell you which engine you have, as well as trim & color codes, and tranny info
that is an example of the plate you are looking for.. that particular one is for an EJ253 - it is the first 5 characters of the "Engine type" that you are interested in. (unless your car has had an engine swap, in which case, this info is pointless) there are multiple configurations of the 2.5, so having that 5th character - letter or number - is pretty important in making sure you get the correct parts for your engine.
Thermostat is typically not a necessary change item, unless you are having temp issues - if you do decide to change it, get an OEM one only - aftermarket are junk. (search the forum here for more details)
This is also a good time to replace the accessory belts, since you will have them off anyway to do the timing, fresh oil change & filter is a good idea as well.
you can find a service manual here to help you along with the job: http://jdmfsm.info/Auto/Japan/Subaru/Legacy_Outback/