Both the 318i and A4 are at the end of tis life cycle. A new model is definitely planned, probably to be launched in 2005/6.
As for the S60, an interim facelifted model is planned for 2005 as well.
The 93 is the youngest of the lot. So it will look "fresh" for many more years than the present other models.
Price wise (base models), from lowest to highest, I think it will look something like this: 93, S60, A4, 318i.
In terms of value for money, the 93 delivers. Just compare the list of standard features and you will know what I mean.
The 93 and S60 have the turbo power advantage over the normally aspirated A4 and 318i. The transmission in the A4 is CVT, 93 is Sentronic, and S60, 318i is Automatic. If you like the choice of both automatic and "manual", it comes standard only on the 93.
It is better for you to test drive all 4 models and make a choice based on your own value system.
I love my 93.