Whats your budget? No one has made a 4 seater since about 2004. There is really no such thing as a forward/reverse gear, per se. All wave runners only have one transmission that moves the prop in one direction. Reverse is typically achieved by dropping the rear gate. But, there are different and more efficient ways to do this. Sea Doo has their IBR system and Yamaha now has one that is more similar to IBR, but they all work by dropping a gate.
IMO, about $4,000 will get you a decent used 4 stroke. Besides the Sead Doo spark, a new four stroke will run $9k at best.