The Dig valve should do what you're trying to do. If you use good 9 volt batteries they should last a year without any problems. The Energizer Industrial 9 volt battery is an excellent battery. I have 7 Dig valve adapters installed at a customers house for the past 5 years or so. We did have the batteries die a couple of times within a year but the rest of the batteries easily lasted the year. The customer now has me install new ones every 6 months which leaves me with quite a collection of still usable 9 volts.
One idea about installing the new valve. If for some reason it's tricky to get that old gate valve out of there you could just open the valve and leave it where it is installing the new valve somewhere else.
It looks like there are two valves there, so you might want to consider something more conventional, if cost is a factor. Antisyphon valve is the proper replacement, if located at the proper elevation.