What does Mrs. Gardiner warn Lizzie about in Pride and Prejudice?
Your original question had to be edited because it asked more than one question. Please do not ask multiple questions in future. I have therefore chosen to focus on your last question.
The answer comes in my edition in Chapter 26. Mrs. Gardiner, who in many ways is more of a mother to Lizzie than her own mother, takes care to warn Lizzie about forming an inappropriate relationship with Wickham due to the lack of fortune on both sides and Lizzie's need to choose a partner not solely because of love. Note what she says:
"Seriously, I would have you be on your guard. Do not involve yourself, or endeavour to involve him in an affection which the want of fortune would make so very imprudent... you must not let your fancy run away with you. You have sense, and we all expect you to use it."
Note how this quote and Mrs. Gardiner warning Lizzie about marrying Wickham are yet another indication that marrying for love was a luxury that very few people could actually look forward to. The economic situation of Lizzie's family indicates that Lizzie must bear money in mind with any possible choice of husband.