I do have my old pioneer 5.2 receiver, will i be able to use that?
For amplification to additional speakers? Probably not the best way to go, but it MAY be usable for two channels of audio. You just have to NEVER touch the volume on that receiver. So, set it to stereo, use the pre-outs from the new receiver into your old receiver and you can get two channels of amplification. It's not typical, and it's not recommended, but it is something that will work.
Better to look around for a deal on a dedicated amp or two. Some of the older Crown gear is super cheap and works very well.
Be aware that the BenQ ST models may lose a bit of focus at the larger screen size, and you MUST pull conduit to your projector location. Do NOT plan on a short throw 4K projector if you can avoid it. I will strongly encourage you to consider JVC if you are truly building this out as a home theater space.
Make sure you look at wall, ceiling, and carpet colors, and DO NOT FORGET TO DO YOUR LIGHTING RIGHT!!!
http://www.avintegrated.com/lighting.html
People mess up lighting all the time during construction which completely bugs me. Multiple zones on dimmers. No main lights should be LED as LED can't dim down all the way properly. Lighting over each row of seats should be on a separate dimmer. You should have at least one light over each seat. Dark rooms soak up light, so use twice the amount of lighting that a bright white room would normally use to achieve good brightness for cleaning and maintenance of the room. You can always turn lights off, but it is tough to add lighting after the fact.