Right now their merchandise selection is configurable but how they actually select from the list, 
how much money they have, and a few other things are hard coded. 
This is actually quite easy to do and I was nearly done with it but I'm not sure where I stashed the changes.

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In doing this I am thinking of just adding some new stuff into the existing .XML files. 
If the new stuff doesn't exist the current built-in behavior would be the default. 
So one of the XXXXInventory.XML files would in the future look something like this (Using Tony as an example):

-  setting Tony to be a repair guy probably will not work
<ARMSDEALERINDEX>0</ARMSDEALERINDEX>  	<!-- 0 to 18 -->
<SHOPKEEPERID>91</SHOPKEEPERID>      	<!-- Prof*.dat / MercProfiles.xml ID -->

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A few notes:
0) You MUST specify ARMSDEALERINDEX if you use any other field.
1) You don't have to update any of the dealer inventory files as they all default to the existing behavior. You can do things piecemeal, too. So if you just want to change Tony's max cash you can change just that.
2) You can't change dealer basic functions (yet). So Tony will buy and sell and not repair things and so on.
3) I don't know what effect changing those ARMS_DEALER_XXXX flags have. Use at your own peril!
4) Changing ARMSDEALERINDEX and SHOPKEEPERID will probably break things in the game. Again, change at your own peril!

The arms dealer indices are:	ProfileID
<ARMSDEALERINDEX>		<SHOPKEEPERID>
0: Tony				91
1: Franz Hinkle			124
2: Keith Hemps			147
3: Jake Cameron			113
4: Gabby Mulnick		104
5: Devin Connell		61
6: Howard Filmore		125
7: Sam Rozen			126
8: Frank			92
9: Bar Bro 1			151
10: Bar Bro 2			152
11: Bar Bro 3			153
12: Bar Bro 4			154
13: Micky O'Brien		96
14: Arnie Brunzwell		128
15: Fredo			130
16: Perko			74
17: Elgin			112
18: Manny			155

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You edit one of the xxxxxInventory.xml files as I show above. 
In your case to just change the dealer's cash. So to increase Tony's daily cash to $45,000 you would do:

<?xml version="1.0" encoding="utf-8"?>
<INVENTORYLIST>
	<CONTROL>
		<ARMSDEALERINDEX>0</ARMSDEALERINDEX> <!-- 0 to 18 -->
		<CASH>
			<INITIAL>45000</INITIAL>
			<DAILY>
				<INCREMENT>45000</INCREMENT>
				<CASHMAXIMUM>45000</CASHMAXIMUM>
				<RETAINED>0</RETAINED>
			</DAILY>
		</CASH>
	</CONTROL>
	<INVENTORY>
	...etc. as usual...

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<CONTROL>
	<ARMSDEALERINDEX>2</ARMSDEALERINDEX> <!-- 0 to 19 -->
	<CASH>
		<INITIAL>10000</INITIAL>
		<DAILY>
			<INCREMENT>10000</INCREMENT>
			<CASHMAXIMUM>10000</CASHMAXIMUM>
			<RETAINED>0</RETAINED>
		</DAILY>
	</CASH>
</CONTROL>

That would change Keith to have 10,000 to start and $10,000 a day

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. Initial is what it is set to at start of game.
. Retained is the % of the previous days $$$ that they retain.
. Increment is what is added each day.
. Maximum is the most they will keep after incrementing.

I did it that way so you could have a merchant slowly build up a fairly significant amount: set init and increment small with drain at some no-zero value plus max large
Or maybe someone who keeps a lot of cash after sales but it slowly drains away.

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