A Tiger without the spinnaker shouldn't be much harder to sail than an H18 (First cat I ever sailed, and I sailed an MC before that).
Adding the spinnaker on light air days is definitely a good way to ease your way in - Sailing the spinnaker in 15-20 is unbelievable, but you gotta re-program your tiller arm as all your instincts will be backwards.
You need at least 2 solid people to right the boat. Its a much bigger boat and a simple matter of mass and leverage.
The comment about expensive repairs is one to consider but shouldn't stop you. You have to be very careful with rudders and daggers - they are expensive and the daggers go much deeper. Used sails are tough to find and generally rather expensive as well. I won't even talk about new ones...
All that being said, the boat is much more efficient, goes faster, and more rewarding to sail. It is an awesome platform to learn F18 sailing. I spent last summer learning the finer points of F18 sailing on a Tiger club boat and finally decided to jump in and purchase an F18 this winter.
As for the boat in question, I've sailed with (against) it several times on Lk Michigan and can vouch for the condition and care that has been taken. I'm sorry to see it for sale, but hope someone in the area will pick it up, enjoy it, and use it well.
'88 H18 "Jolly Mon"Sail Michigan's Great Lakes in 2013cramsailing.com