Thanks in advance.
Here are some general suggestions. I don't know exactly what character level you want or your opinion on the defining characteristics of the "Punisher". Since D&D is set in the medieval period and the Punisher generally uses modern firearms some latitude in equipment design is required to avoid an anachromism.
CLASS:
The Punisher has no superpowers, but is extremely robust, skilled with firearms, and has a tendancy towards brutal tactics. The best class for a "Gung-Ho" Punisher would be FIGHTER; although the Barbarian (for his rage) or Scout (for run-and-gun attacks) are additional possiblities. The fighter provides the most combat feats and so allows for the most customization of fighting and shooting tactics.
Since D&D is a universe filled with magic and supernatural menances, an "URBAN RANGER" is another good choice. As favored enemies he might choose his own race, devils, or other monstrous tyrants (Lawful Evil races). Choosing the archery track will allow the ranger to feather his foes from afar and grant him targeting feats when wearing light armor (like the T-shirted Punisher).
A ROGUE may be the best class used to represent the Punisher even though its Hit Dice and BAB are low. Sneak Attacks allow for one shot kills and the Evasion and Uncanny Dodge would allow the character avoid attacks that would destroy ordinary mortals. The hand crossbow (which the rogue has class proficiency) is similar to a pistol. The sneakiness of the rogue corresponds to the commando tactics of modern soldiers.
STATS:
The Punisher is not super strong of super dexterous so his attributes for STR, CON, and DEX should be 14 to 18. All his mental attributes should be average to poor (9 or less).
FEATS:
I would consentrate on weapon feats. One of his signature moves is to fire a automatic rifle one handed but I don't know of a D&D feat that coresponds to that. "Monkey Grip" come close which allows the character to weild a weapon one size larger than he can ordinarily use. Feats that increase rate of fire will be prefered: Rapid Fire, Multi-shot, Two Weapon Fighting (applies to ranged weapons), and Quick Draw would be good choices and reflect the autmatic weapons the comic Punisher often uses.
LEVEL:
The level of your "Punisher" character will equal its CR and will determine the number of feats he has available.
WEAPONS:
D&D does not usually use guns. A small selection of modern firearms is presented in the DMG. The closest correspondant to firearms in mundane weaponry are hand crossbows and repeating crossbows. Wands and Staffs are the magical equivalent to firearms but require spell casting ability or the rogues "Use Magical Device" skill to activate.
PUNISHER CHARACTER STATS:
After saying all of the above this is my best stab at recreating the "Punisher" as a 10th level rogue. Adding two levels as a Fighter will boost Base Attack Bonus by +2, 2d10 Hit Points, +3 Fortitude, 4 skill points, and grant two additional fighter feats (point blank shot and rapid shot are good choices). Adding one level of Barbarian will add +1 BAB, +1d12 HP, +2 Fort, 4 skill points, and grant the "rage" ability once per day.
FRANK CASTLE, male human Rog10, CR 10, Size M (6ft 4in tall); HD 10d6+40; hp 78; Init +7 (+3 Dex, +4 Improved Initiative); Spd 30ft, AC 13 (+3 DEX); Attack +9/+4 melee or +10/+5 ranged; SV Fort +5, Ref +9, Will +4, AL LE, STR 14, DEX 16, CON 18, INT 11, WIS 13, CHA 9
Class Abilities: Sneak Attack +5d6, Trapfinding, Trap Sense: +3, Evasion, Improved Uncanny Dodge, Crippling Strike
Skills: Climb +14, Disable Device +11, Hide +15, Intimidate +10, Jump +10, Listen +13, Move Silently +16, Open Lock +13, Search +12, Spot +1, Use Magic Device +11.
Feats: Diehard, Endurance, Exotic WP {repeating crossbow}, Improved Initiative, and Quick Draw.
Possessions: 16,000gp in gear. Likely weapons such as enchanted hand crossbows and repeating crossbows and inconspicuous and stealthy armor. The Punisher will make use of explosives. D&D explosives can be special crossbow bolts, alchemical grenades, or "fireball" scrolls, wands, and necklaces