How do I choose the correct params? I am very new to airplane design. I am working on this project to design a flying wing glider (no propeller) that would glide in a circular pattern with a fixed radius of 250 m after being released at a height. 
Here is all that I know now: 
I was reading around the internet that a choice of delta wing would be good for maneuverability of the glider, however it would be bad for operating at low speeds. I guess that this design would not fit me because it has very poor lift. But at the same time a wide plank kind of design would not be able to perform a turn that well. So I've decided to choose a swept wing for the flying wing. However, I am not able to decide properly the angle of sweep required. I know that the stall angle and the twist of the wings depend upon it, but I don't really know how to tackle the problem (as I guess I would require a high stall angle to provide me the lift).
When it comes to choice of airfoils, I am currently looking at HS522, MH60, and NACA 22112 and Eppler 186 airfoil. However, again I can't really choose between them. From my understanding, I require a reflexed airfoil with high camber and high thickness to provide me with good lift as I plan to operate at slow speeds close to ~20 m/s.  
Wing loading and banking of the turn is another thing that I understand is related to each other but I don't understand how I can reach a common point between them. From my preliminary analysis of the situation, I have reached a bank angle of 9°. Is that a good angle? I read on online resources that the standard bank angle is 25°. Is that relevant in this case? Or do I approach this problem by deciding the wing loading and then move on to the bank angle?

Thanks
A flying wing needs an airfoil with an s-bend, so no regular airfoil (none of the ones you mentioned). And you need to incorporate a significant twist towards the wing tips to make it fly stable (incidence is about 10° lower at the tip).

Knowing your turn radius and your desired speed your bank angle is a given, independent of aircraft mass. You can compute it with the centrifugal force and the lift portion that points into the turn... 

r = TAS * TAS / ( g * sin( bank ) ), with true airspeed (TAS) in m/s, g = 9.81 m/s/s, r in meters. (if I'm not mistaken)

When you know the mass of the aircraft (roughly) and your airfoil lift coefficient that you want to work with (e.g. 0.5 to 0.8 or so) you can work out how much wing area you'll need to fly that turn without stalling. You could also chose a wing loading and compute the lift coefficient for your turn using the basic lift equations and the force equilibrium...

25 degrees is a good bank angle for passenger aircraft, more makes the passengers scared :) Not applicable here.

I'd start with an existing design of a flying wing and then make alterations to that. A famous aircraft that comes to mind is the Horten H.IV glider. Another example could be the Marske Pioneer 3