Mod to get siege equip to battle on time

I want to mod the game to get Siege Equip. to the battle site at the same speed as the rest of an army. I have tried many approaches to this, and looked as deeply into the files structure as my poor skills will allow. No method of file modding seems workable to me.
But I have decided on a work-around approach. I intend to use the mercs capability to get the siege equipment to the battle site.

I am going to use scripting to set up conditions for the availability of "buying" siege units at the battle site the same way that merc's are bought.-but I will try to set it up so that the player views it as the arrival of the siege engineer gives the general the ability to build the siege equipment on site - and if this works well, later I will do another script for cannons, which obviously cannot be built on site. ( The test will include the availability of siege engineers in the Generals entourage, the existance of siege works at the place of origin of some of the besieging armie's units - perhaps the general's place of origin - and any other test that would seem logical and appropriate to the issue.)
As my skills in scripting are nill, and as I have only done one successful mod - a mod to increase the (%) odds for some of the entourage charaters' movement points, tax collection, and other such effects, - I have no good idea as to which files I will need to mod in order to effect the siege equip. issue.
That is where I am asking for your help. Here is a list of files I think I need to mod in order to get the thing to work. But I need you to tell me if there are any other files that will need to be modded.
TEST: When are siege (merc's) available? = events.txt : mod script to test for conditions.
MOD: Which units (siege type) are available? = mercenaries.txt: add siege equip to the merc's list.
MOD: Define the unit details for each piece of siege equip ( This may have already been done, as the equipment already exists, but I need to make sure that each unit can be put into the merc's group.) Cannons may come later, with a different critria and script set.
What other tests or files sould I be looking at?
What I need you to help me with is to let me know if there are any other files which need to be changed to get this to work. I am hoping that no texture files need changing, as that is far beyond my current skill set.

BTW: I know that when I did my entourage mod and increased % tax collection, movement, and some probabilty odds, that my changes to the ancillary files did not change what was shown on the actual screen; although the various changes did work. But the pop-ups for each, showing their % or + movement, or -+ loyalty did not change form the original text...so it is obvious that the changes to these factors did not "blow through" to the actual game pop-up text. E.g., I change some of the entilement tax collections from 5% to 10%, but the pop-up still shows the 5%, although the character clearly is collecting at 10%. I do not need your advice on this just yet, but some help on the siege equip issue would be great!

Regards, Greg.