The temperature of the water will ofcourse depend on the depth from which it has come up from. If this is fairly shallow then it is likely to be about 10 degrees C and vary seasonally. If it's from fairly deep then it's likely to have a temperature of upto to about 14 degrees C.
If the well is still open then the heat exchanger could be installed down the well, thus not relying on flowing water.
If you want to measure the flow then generally the type of weir depends on the range of likely flows.
For low flows (up to say 10 l/s) a 28 degree V-notch weir would be more suitable. For higher flows of up to say 60l/s a 90 degree V-notch weir would be more appropiate for flows in excess of this then a rectangular notch would be more suitable. The most common type we use are 90 degrees has these give a large range and are fairly easy to cut.
There are a few published pieces on getting geothermal energy from mine water discharges etc.
Another popular option for a similiar type of heat extraction is via laying of load of pipes under the topsoil and getting heat from the ground.
I went in a mine once.... it was dark and scary..... full of weirdos
When do I get my soapbox, I need to rant on about some b***cks