Please keep in mind, if you are just using this for casual movie watching and a first time experience, then you have to consider how much work you really want to put into this setup. There are some very deep enthusiasts here who will talk at a very high level, and that's not bad, but also isn't always helpful.
Going with a less expensive receiver, right now, allows you to see just how into the whole surround sound setup you really are, and how far down the rabbit hole you want to go. A 4K (or 8K) capable AV receiver can be sold within a couple of years for a reasonable price on eBay/Craigslist and then you can get something MUCH better if you want it, and budget allows, or you may find that you just are perfectly happy with what you have.
An important consideration here is that Atmos speakers aren't just stapled to your ceiling. It can be a significant investment in labor to get wiring to the ceiling locations for proper speaker placement. They are not wireless, so getting those wires there, especially if you want them run in-wall, isn't going to be within budget unless you do the work yourself, which may be easier said than done.
I like good receivers. But, even though I absolutely KNOW I will run a full 7.2.4 Atmos setup in my final theater setup, I ended up only getting a 5.1/7.1 capable AVR when I purchased a new model last year. The cost jump to get a full 7.2.4 capable AVR was far more than I want to spend right now, and when my theater space is finally ready and capable of the full Atmos setup, I will dump what I have and spend the extra money necessary on the better receiver.
Keep in mind how many channels, what location, and what quality you expect as well. I will say that a 7.1 channel receiver isn't that expensive and can give you a 5.1.2 Atmos setup without it costing you an arm and a leg, but it really is about what you are hoping to achieve this go around. Receivers really do tend to give you more product as your spend more money. You get more inputs, more connectivity, more power, and more channels to work with. Features improve as does build quality (typically). But, it is very much going to be a reflection of your needs.
I don't think the Best Buy guy was right, or wrong. He had a discussion with you and made a recommendation. It's great that you checked here, and you may want to even get some opinions elsewhere about your setup. But, it does come back to cost, expectations, and desire. But cost is first. You need to find out how much pulling cabling from your AVR location to all those speakers will run. As well, if it is behind drywall, and there is no access, then holes will need to be made, repaired, repainted, and it could easily cost over $1,000 in labor to get those cables in place, which may not be a part of the budget this year. Installing speaker wire and ceiling speakers has almost ZERO difference in labor compared to installing recessed lighting. Wire must be run from a specific location to the ceiling, around and through joists, and the goal is minimal drywall impact, but the installer isn't going to fix any drywall cuts which need to be made.
There are some budget AV receivers which may be worth considering... The Yamaha RX-V6A for example is a bit less, with similar feature set to the Denon.
Or, certified refurb models without 8K support, but full 4K support can match the higher end Denon in many areas, yet be half the price...
www.accessories4less.com
If your end game is a full 7.2.4 Atmos setup, then that may be well out of budget at this point.
SPEAKER PLACEMENT IS EVERYTHING! With all else said, throwing speakers into your room but not getting them in the right locations is a complete waste of time and money. Buying a receiver without any intention of spending the money to get speakers installed where they MUST go for proper sound is also a waste of money. If the goal is all 'several years out', then I would go with the idea that the receiver may be a temporary fixture in the home that will need to be replaced.
Still, if I wanted a full 7.2.4 setup in the near future as an option, I would be hard pressed not to go to Accessories 4 Less and pick up a Factory Refurb receiver like the Denon 3600 for $1,000. It's a bit more, but has all the bells and whistles needed for a full 5.2.4 setup out of the box, and a 7.2.4 setup if desired in the future. I would have a hard time buying a brand new AVR at current pricing unless I was specifically going 8K and needed those extra bells and whistles.
