Hi babynus!
I usually plan my pre-sales projects in the way you mentioned, not only in Projeqtor but also in other portfolio management software.
My two cents: I think that the tricky thing could really be to add some fields and some ways to calculate the effort estimation for each activity.
The calculated estimation will be used in the schedule baseline of the project once it is approved. The difference is that, due to the resource availability, the effort will be translated into duration and elapsed (as Projeqtor already does while scheduling considering the time the operator inputs in the activity progress).
Here there's some consideration about.
For each activity, Projeqtor could provide three estimations: optimistic (OE), pessimistic (PE) and most likely (ML). These three estimations for each activity should be mandatory (at least two, read below).
Then I imagine three ways to estimate for each activity
1) three points estimation: (OE+PE+ML)/3 . This is the most used one, but less accurate. At least this should be developed in a first moment
2) weighted: only two estimations, OE, and PE, but weighted with an integer factor (WE), which goes from 1 to 3. The algorythm is: (((PE-OE)/3)*WE)+OE. Largely used with a little bit more accuracy
3) PERT estimation, based on BETA distribution: the most accurate. The algorythm is (4*ML+OE+PE)/6
Obviously the cost estimation will be performed when resources and their cost will be assigned to the acivity, otherwise you won't be able to get any cost estimation for the activity or the whole pre-sales project.
Finally, the MonteCarlo analysis, which is an estimation "by project" could be applied to all the project activities ON THE CRITICAL PATH in order to have the most accurate forecast in terms of time for the whole project.
MonteCarlo distributions represents just the probability to end a project (or an activity) before a certain date and is made by casually iterating the calculation of effort or cost of each activity in the project according to the method you prviously selected.
MonteCarlo can be applied also to costs, but in this case it must be applied to the whole set of activities in the Project.
The difference between a sales project and a real project is that in the first case resources are considered to be unlimited (each one at 100% on their activity), and the project won't have any lag.In other words, only effort should be considered, as said at the beginning of this post. Estimation methods and MonteCarlo could also be applied when re-planning.
That's all.
I understand that's a lot of things, but... maybe it deserves some investigation...

Have a nice time!