Of the three major GPR manufactures (GSSI, Sensors & Software, and Mala) I am biased towards Sensors & Software. They make very good equipment that is reliable and well thought out. They also have very good GPS integration, better in my opinion than GSSI. We just bought a new Sensors & Software PulseEKKO Pro system with most of the low frequency antennas (10 MHz to 500 MHz) and the cart for about $80,000. It isn't cheap and requires some technical know how. Before I would tell anyone to buy a GPR system I would suggest that they get some experience with a knowledgeable user or a training course. Also buy borrow the following two books:
Conyers, L.B., 2004. Ground-Penetrating Radar for Archaeology, edn, Vol., pp. Pages, AltaMira, Walnut Creek.
Daniels, D.J., 2004. Ground Penetrating Radar. in IEE Radar, Sonar and Navigation series 15, pp. 726, eds Stewart, N. P. & Griffiths, H. The Institution of Electrical Engineers, London, UK.
The book by Conyers has almost no math but will conceptually help you get an idea of whether GPR will help solve you problem. You could also try out GPRmax (
www.gprmax.org) modeling software to model your target.
I would caution against the idea that GPR is a silver bullet of some sort. As I have said in many other posts, each geophysical method is a tool and may or may not be applicable to your problem. Buying a rather expensive piece of equipment may limit the number of tools in your toolbox. All geophysical equipment is available for rent, as many practitioners don't own any of their own equipment.