Note of caution - you pay for what you get. Yes, you might find that an online hearing aid site can undercut a dispenser by a large margin. National chains like Specsavers and Boots can be cheaper than certain audiologists. You will also find that certain dispensers/audiologists charge more than others. You might find that those with expensive premises may charge more, and that location could be a factor - i.e. Harvey Street in London is likely to be more expensive, and the north may be cheaper than say, the south east of the country.
However, once regional and other differences are accounted for, in my view, you have to be very careful about being attracted to low cost. The problem is whether you are paying for after care, and also - the skill of the audiologist. If you are are paying little more than the wholesale value of an instrument, then you might be getting no aftercare and little skill. Hearing aids are not as easy to fit as say, spectacles. Chances are that you may not have a successful first fit. You will need to go back and make adjustments. In my experience, certain dispensers (low cost), want to move on to the next customer - and I hope this is not a slur on them as a whole, who after all, are just making a living. If it’s online anyway, you have a problem - how are you going to make adjustments ? If it’s a national chain, with local reps, then they will be peripatetic and I have found getting their time to be a problem. An excellent audiologist, near me in Cambridge, charges double, but for that you will get proper testing (REM), a rigorous first fit and open ended time in adjustments. If one considers that you are getting the full attention of a professional - and given that most experts I am aware of in engineering/IT/legal fields etc etc, will charge at least £500 a day, then arguably the money is well spent. It really depends on if you think you can get away with having your hearing aids programmed quickly, with minimum fuss, or whether your hearing is going to require more expertise. Right now - I have learned the hard way with certain dispensers. I hope that hasn’t offended anybody, as I appreciate that dispensers/audiologists must be at the mercy of manufacturer’s first fit algorithms etc.
On your other points, you need to find out what tests etc they offer. They will not charge you for a more expensive model and give you a cheaper one - that would be fraud. You will know what the model is - generally, because hearing aids are all branded, even within those chains that have their own brands like Costco or Specsavers. Don’t really understand your question re: trials. Most dispensers have a free 30 day trial with some extending that period further; you are under no obligation to buy after that period; you can walk away.