Nature of Contracts, Agreements, Consideration, Capacity, and Genuineness of Assent

Please help me to understand  these issues presented and help me to understand the issue, law it's relevant to, how to apply it to the law, and what conclusion can be drawn from them.  Thanks.

SIDING:

Juan owns a house in a poor area of a large city.  A salesman visits his home, selling aluminum siding.  Juan buys, and signs a contract, which calls for a price of $10,000, to be paid in monthly installments of $500 for 5 years.  Juan only earns $1500 a month. In addition, Juan's home is only worth $50,000; the aluminum siding put on is worth no more than $3,000; Juan speaks and reads very little English; and Juan though he was signing a receipt for a free gift.  

BILLY KIDD:

You buy a pair of very used skis for $25 from a sotre in Steamboat Springs.  You learn later that the skis were owned years ago by Billy Kidd, a famous skier.  The store demand that you return them because it did not know that they once belong to Billy Kidd.  

STUCCO:

Bob buys wall paint that works fine for all walls, except for stucco, which is what the buyer bought it for.  The can simply says "wall paint," and contains no express warranties or disclaimers.  He sues the store and the paint manufacturer under breach of implied warranty, and negligent misrepresentation (failure to disclose).  

YOSHI'S:

Yoshi's fast food restaurant had a program: one can get $3.00 off a meal after one has bought 10 previous meals as evidenced by ten stamps on their coupon card.  Is that a unilateral or bilateral contract, and what difference does it make?  What if after 5 stamps, Yoshi's changes its policy and discontinues the programs. Breach of contract?