The best description - although it is a bit self serving and salesey - is at http://javelintrain.com/
If you watch the video at the bottom of the page it shows the Javelin, or also known as a Class 395, or HS1 (for High Speed 1) departing from St Pancras on the same platform that your train will arrive on. The Javelin is the dark blue train; on the adjacent platform you can see a waiting Eurostar (white with yellow underneath). The ticket barriers are just visible at the head of the platform.
Changes at Ashford International are step free (except for the usual steps down from the train and up into the train) because of lifts to all platforms. That is from memory because National Rail is having a few glitches at the moment. You may very well not have to do more than off one train, wait 10 minutes and onto the next one on the same platform.
I wish you had mentioned your husband's disability at the beginning.
All you gave us was that you are seniors (so am I) and "will have heavy luggage and can't walk a lot" which is common to many cruisers.
If he is disabled you can arrange for you both to be met by staff to assist you on and off the trains at Dover, Ashford and St Pancras (both services). They can often assist with one or two small bags, but the one person won't be able push a wheelchair and handle lots of heavy luggage, and they have limits of the weight they can lift on and off trains. I hope that all your luggage has wheels?
Each railway company has a different assistance contact number, and they all require minimum 24 hours notice for assistance.
If your husband can't do the steps up into (or off) the train the assistance person can fit a ramp - the same if he is in a wheelchair.
You may find that the Ashford change is pretty easy, and with the Javelin terminating at St Pancras, and the Eurostar originating there, it will give the assistance folks time to do their job.
There used to be direct services from Dover Priory to St Pancras and there was an 08.xx something direct to St Pancras when I looked this afternoon, with the remainder a 10 to 13 minute challenge. With the National Rail hiccups this evening I can't check.