If you are buying a quality piece of home audio equipment (ie. Name Brand : Denon, Onkyo, Marantz, Pioneer, Yamaha, Klipsch, Mission, Bose, NOT F*CKING SONY or SAMSUNG) it honestly should last you at least two decades if not longer.
The tricky part is finding an actual 2.1 channel unit these days.
Everyone is pushing 5.1 and 7.1 home theater setups these days that would bankrupt you in speakers.
Seriously, unless you're buying a cheezy all in one unit, you should be able to walk into a better class stereo shop and walk out with a decent setup that could do the job of voicing the keyboard as well as entertaining the family. You don't need more than 80 Watts per channel for home use. That provides enough dynamic headroom at reasonable listening levels that you don't start clipping the signal when a cymbal crashes or a THX commercial goes extremely loud. Distortion numbers these days on good equipment is typically 0.005% (half a hundredth of a percent) or in that neighbourhood and pretty much all amps sound equally good with those numbers. Speakers are the weakest link in any audio chain with numbers typically around 1% or 2%. The only way to possibly get better numbers is to buy laboratory reference speakers. (Mucho Bux!!!)
If you want to be able to play music on a tuner or CD while playing your keyboard at the same time, you need a receiver or integrated amp with a tape monitor function. Nobody uses tape any more but this will allow you to connect your keyboard thru a small mixer as well as your computer and the stereo's programming all thru the same input on the amp and control individual levels independently with the mixer.
If you simply plug the keyboard into the aux input then you wouldn't be able to play along with a CD or other program unless you fed it thru the mixer built into the keyboard. Line levels are the same for professional keyboards, drum machines and modules as they are for home stereo equipment. All that is required is the proper cable or adapter to connect pro audio gear to a home unit.
A decent home stereo that would bring Carneghie Hall to your living room should be doable for about $1000 USD all in.
I lean toward the Klipsch because I recently acquired mine and have noticed the clarity and improvement first hand as well as because they are currently easily available and at a great price as well if you take advantage of one of Best Buy's never ending sales.
The decision is up to you but as your setup evolves and you start looking to record and play back with true fidelity, you pretty much HAVE to become a DIY audio engineer
Gary