Well, with "casual use" and "nothing high end", then any old splitter will do. You can find perfectly good splitters at Home Depot and Lowes that will suit your needs perfectly. Make sure you get a 2-stage pump. That means that with low load, the piston extends quickly until the ram engages the wood and the load is high, then the pump automatically switches to 'low gear' to push through the wood. A single-stage pump goes slow ALL the time.
Don't bother building one. It costs SOO much more, and you likely won't be happy with the results, especially for a casual splitter. Save your time and money and buy new.
Avoid used, unless you know what to look for in engines and hydraulics and can spot problems and know how to tinker with it all.
My own personal recommendation is a SuperSplit. It's more high-end, but I split about 10 cords a year, which might be considered 'casual' by some. Instead of the 12-second cycle time of most hydraulic splitters, this has a 2-second cycle time. Much faster and uses less fuel. Don't bother trying to find one used - these things are rarely available that way! The owners tend to really hang on to them.