Components on floor

No issues. I had amps on the floor. Much better sound than having it all in a rack. Spread the components out and you’ll be fine. If they are at least 1 foot away from the fireplace you’re good. However if they are on the same circuit as the fireplace you might get some AC line noise. See how it all sounds. 
 

I have all source components on a DIY decoupling shelf system. I have suspended floors. My subs sit on Stillpoints SS, my monos sit on a Symposium Ultra each with HRS footers. This does a great job. I agree that a spring setup under wood platform would be A very effective solution.

I’m thinking of about a Dimplex IgniteXL 60" fireplace unit that’s either on or off.  Instead of mounting it to the wall, the carpenters will build a cabinet to hold the unit at the top with a shelf at the bottom for audio gear.  The cabinet will abut the wall behind with slots for ventilation.  The floor is hardwood over concrete.  The walls are furred-out studs and wall board over concrete.  The gear is an Ayre DX-5/20 disc player, QX-5/20 DAC/preamp, AX-5/20 integrated amp, and Roon Nucleus.  I gave my Thorens TD124 and vinyl to my cousin.

I’m 89 but can comfortably bend to insert a disc.  Mostly I stream music or watch YouTube/Netflix on the 85" Sony. 

Don't put them on the floor. Use amp stands.

I’d say lord in heaven no if not a concrete floor.  Even so elevating a bit makes use easier.  If foundation level and any chance of flooding,  definitely no. Distance from any nearby electric devices always desirable just in case.