Berserk is absolutely awesome. If you haven't seen them (the anime... I'm not familiar with the manga), you must.
If your PC is level 10, I would argue that the level 10 Guts (Gatsu) hasn't lost his arm yet, and therefore wouldn't be eligible for the repeating crossbow/cannon combo, though maybe that's irrelevant. I think someone suggested the gnome artificer from Magic of Faerun. Fireball, scorching ray, or orb of fire might be appropriate. I'd use those rules somehow. I wouldn't bother with ammo and gunpowder. Exotic weapon prof. might be in order for the modified hand crossbow hybrid.
Though it is a prosthetic arm, I'm not sure how much use he has with the hand. It's magnetic, which allows him to sort of "stick" his sword's handle to his prosthetic, allowing the use of both arms for the full power attack, but I don't know if Guts can manipulate much with it. I'm not even sure if he can use his crossbow or hand cannon without using his other hand to trigger the bolts and/or pull the firing string of his hand cannon (which he actually pulls with his teeth in one of the early episodes of the anime).
As for his signature sword(s), my buddy and I discuss this often. I suggest, as others have, monkey grip + huge greatsword. I might suggest adamantine to reflect its clear durability during the series.
As for levels, I'd endorse levels in Kensai from Complete Warrior and maybe Weapon Master from Sword and Fist, as his abilities with his sword are unique to put it mildly, and later in the series, Guts appears to devote his life to wielding his sword, living for the joy of the sparks created by his sword battles, claiming once, "I'd rather fight for my life than live it." He becomes almost spiritual about his sword, taking an "oath of service" if you will, which to me suggests Kensai. Certainly the Kensai ability power surge seems to fit nicely with Guts' "rages" if you will, though I don't believe he ever rages as a barbarian would, not even during the hundred man slaying, and his madness during the God Hand ritual was exactly that: madness, in my opinion. I also don't think Guts ever lived the life of a barbarian.
Of course, Guts' oaths (if that's an accurate term) to the Band of the Hawk and Griffith could serve as oaths of service as well, fulfilling that requirement.
Alignment is another debate entirely. I'm against him being good, as his actions against Lord Yurius and his family, in my opinion, prove that he is not good-aligned. I don't believe Guts is evil, though. Griffith is evil, but not Guts. I'd ballpark lawful neutral (if only for the Kensai levels in the end) and leave it at that before I drive myself crazy trying to pinpoint an alignment.
Obviously his Str, Dex, and Con should be high. I've never seen Guts in full plate. His heaviest armor might be breastplate, so maybe set his Dex at 16 max. You'd want Int of 13 for Combat Expertise if you like. Str might be Guts' primary ability score.
Guts, Band of the Hawk Commander, lawful neutral male human Fighter 6/Kensai 4
1. Monkey Grip (Complete Warrior), Weapon Focus (greatsword), Power Attack
2. Cleave
3. Combat Expertise
4. Weapon Specialization (greatsword)
5.
6. Improved Sunder, Great Cleave
7.
8. Improved Critical (greatsword)
9. Combat Brute (Complete Warrior)
10.
Maybe slip in Exotic Weapon (repeating crossbow) in there somehow.
