You can use the lower receiver with a collapsible stock but it needs a new receiver extension, buffer and spring. Not a hard thing to do but you'll need (or borrow) a $7 stock wrench. It's probably best to buy a complete kit with matching slider as the slider you now have may not fit the RE you buy (they come in milspec and commercial diameters with milspec preferred).
But now's the time to figure out just what exactly do you have so get it out and shoot it before making changes. I'd leave the rifle stock now in A1 configuration since as you use it you may want a more modern flattop upper. If that's the case, there are people who do like the older design but wouldn't like it with a telescoping stock.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek