Which type of model to use and what's my target variable
I would say the variable 'spent' is your target variable. I'd suggest you to analyse the distribution of each feature and target to check if, eventually a linear regression could be appropriate.
You might try different other methods however, like a decision tree regression, which does not require any assumptions as it doesn't estimate parameters.
What I would suggest you to do, howeveer, is to concentrate on the distributions of parameters to check for eventual unbalances and make groups to make estimation robust.