Crafting a target variable for propensity to purchase My goal is to predict when a prospect customer has the intent to purchase a product (industry has a very slow sales cycle). This is ultimately a marketing model, so predicting/estimating when the prospect becomes interested in purchasing this type of product is as important (if not more) than knowing the actual year they will purchase.
I think my question will be best solved by generating some fake data so I can best explain my intentions. Please see the R code below for generating some fake data.
# setup fake dataset with no purchases or flags to begin with
sales_data <- expand.grid(year=2004:2011,prospect=c('A', 'B', 'C'), 
                          flag=0, purchased=0)


# quick function to populate the sales, nothing to write home about
make_fake_sale <- function(data, prospect, salesyear) {
    # flag our actual purchase (pretend that this has been done already)
    yearindx <- which(data[,1] == salesyear)
    prospindx <- which(data[,2] == prospect)
    prosp_yr_indx <- yearindx[yearindx %in% prospindx]
    data[prosp_yr_indx,4] <- 1

    # set our fake target variable "flag" = to 1
    flag_yrs <- (salesyear - 3):(salesyear - 1) 
    yearsindx <- which(data[,1] %in% flag_yrs)
    pros_flgyr_indx <- yearsindx[yearsindx %in% prospindx]
    data[pros_flgyr_indx,3] <- 1
    return(data)
}

sales_data <- make_fake_sale(sales_data, 'A', 2010)
sales_data <- make_fake_sale(sales_data, 'B', 2008)
sales_data <- make_fake_sale(sales_data, 'C', 2009)

sales_data

that leaves us with the following data below (I cut off prospect 'C', but you get the idea).

(not pictured here are the features I would use to actually train the model itself)
My question is: Is there any danger to removing the 'purchased' column and using this new 'flag' field as my target variable for modeling?  
Are there certain models where this would be a good idea and some where it would not?  
Would it at all be a good idea to maybe turn this into a regression and make up a scoring construct so the year before the purchase would receive a value of 100, then two years before the purchase would receive a 75, and so on? That way we take into consideration how close they are to making the actual purchase? 
The ultimate goal here is to model/predict when prospect customers have started considering their need for a product that has a very long sales cycle (typically over one year).
Also a final note, I am being very careful with how I split my train and test for the validation phase of modeling, so I have already considered the pitfalls there (data leaks, prospects who straddle the cutoff year, etc.)
EDIT: clarifying intentions for this model
EDIT: the industry I am dealing with is low-volume, so the "purchased" column is very sparse. Additional reasoning for the "flag" target variable would be for giving the predictive model more actual positive results to train with.
 A: It is perfectly valid to create a target variable like flag, you just need to be aware of how this differs from the purchased variable and your knowledge of the sales industry is needed to decide which is better.
Obviously flag and purchased are closely related but they both have potential drawbacks. 


*

*Predicting purchased will only give you 1-12 months notice before the customer is expected to buy the item. This might not be long enough for your to market the item to them.

*Predicting flag should give you a few years notice for when a customer will buy a product, the downside is that 3 years notice is treated the same as 1 year notice (you know better than I if this is important). Are customers actually expected to show signs 3 years in advance? Is a customer who will buy in 3 years more similar to one who will buy in 4 or 2 years? If 3 years is more similar to 4 years then treating 3 years notice as a true flag will obscure the pattern in the data.


A possible solution to the issue with predicting flag is to reduce flag gradually between 1 and 4 years ahead of purchase. Perhaps 1 year ahead of time then flag=1, 2 years ahead flag=0.66, 3 years ahead flag=0.33, 4+ years ahead flag=0. This treats 4 and 5 years the same, you can decide if this is a fair assumption.
Treating 1 year ahead of purchase differently from 3 years ahead of purchase allows you to prioritize resources to those customers who will be purchasing soon.
