If its a frag tank, your better off connecting it to the same system as your display. This way you do not need to buy filtration and sump for this tank. Just get it a decent light and some flow. Spend your money keeping your display in great shape, that is what the hobby all about and what people want to look at.
What do you plan to frag and why? If it for overflow/excess coral from your tank? A place to allow frags to grow out? Or are you trying to frag and sell coral to supplement the cost of the hobby. The latter takes a lot of work and is not a good plan as its harder than it sounds.
Better option is just buy or DIY a shelf in your display to let frags grow on it until you permanently mount them or trade/sell them.