I installed a 60 gallon Super stor off of my boiler in place of the tankless coil and am very pleased with the results. Super store also makes a tank for solar which is backed up by a boiler (I don't think the fuel source matters), this would probably work for you, install it now on your boiler, then when you are ready you can hook up the solar. Solar is expensive to install, but oil isn't cheap any more either.
Good luck.
http://www.htproducts.com/superstorsolarrange.html