Abs smell can be anything from unnoticeable to awful with the difference mostly being in the size of the print. Longer prints will smell a lot more than small ones. Very small prints won't smell at all unless you're in a very small room and/or have a very sensitive nose. I'm about an hour into an 8 hour abs print right now and I have my windows open despite the 35 degree day because the smell was giving me a headache.

2 heads are not really useful especially at your level of experience. In my experience so far they cause more problems than they solve. Oozing with dual extrusion prints can ruin a print and even if you're only using one of the two heads, the other can end up scraping the part as it passes over and ruining the print. The benefits are multicolor prints and using different materials for support that can be removed with chemicals that won't effect the main print material. As you gain experience you may want to try it out and at that point you may regret only having the single extruder but for now I wouldn't worry about it.

Not having a heated bed imo is an issue. It limits your material choices dramatically and can also make keeping the prints stuck to the platform more difficult even with pla. Again, for now it probably won't be a problem but I suspect you'll be wanting a heated bed long before you want a second extruder.

Assuming the dremel version is in fact a dreamer or at least adheres to the same level of build quality you will probably be happy with your purchase. If you're a tinkerer at all you will probably find yourself modding it in fairly short order to make it better and/or looking at a second printer to give yourself more capabilities.