Yes indeed- you have discovered correctly that the majority of newer HDTVs have poor to average built in speakers ;(.

If your hearing is similar to my own father's ih that a typical Netflix feed is too low for you to hear, then yes. You should definitely obtain some sort of compatible external speakers, soundbar or otherwise to improve your ability to hear the action ;).

What's 'affordable' depends on one's budget, clearly. Let us know how much you feel like spending. There may be some external speakers for $50 that could suffice, but a lot of what may suffice is subjective to the listener.

I had to look up the Widex TV-Dex. Interesting device. I have a feeling though that you might be able to get it to work with a little jury rigging. Please provide the actual model number of your TV so we can be 100% certain of your options. Basically, I see that the 55" size of that series seems to include both audio minijack and digital optical audio outputs. That means you could possible utilize a slightly different method than what's depicted in the TV-Dex manual to hook up the Widex. See here for reference:
http://www.widex.com/WebFiles/9%20514%200130%20001%2001.pdf

If your HDTV model can send out analog sound, Instead of the Red/White to 3.5mm plug, you would substitute with another 3.5mm to 3.5mm cable (another should have been included in the Widex box). If the HDTV only sends out digital sound, it would be either HDTV digital optical out (using a digital optical cable) -> digital/analog converter box (~$20-30 on Amazon) -> Widex unit. Then you would go out from the Widex unit using the 3.5mm connector/cable as depicted to your audio system (etc.). Otherwise, the hearing aides should pick up the signal w/o the last connection if you aren't sending it to external audio.

I found another person who inquired about a similar issue. They left the discussion too soon IMNSHO. Read here:
http://www.justanswer.com/home-theater-stereo/7lqhi-recently-purchased-denon-avr-1712-several-devices.html