Well, my personal favorite start for any new character is this:
- 'rough and mysterious figure' -> 'left you to fend for yourself' -> 'became a bard' -> 'a series of unfortunate events'
This will net you 9 in Looting and 6 in Trading right at the start. This is a very strong boost for the early phase of the game, as you'll be able to loot decent equipment for you and your companions, and sell the rest for hefty sums. And you'll start with 3 in Power Throw, which will be a blessing in tournaments with throwing weapons, without the need to invest in throwing. Another good thing about this setup is that you'll get one of the highest rumor value possible. This means that you'll get a bigger chance for unique spawns and events to spawn/happen. Also, if I remember correctly (I don't have a computer in front of me atm), you'll get a winged mace to inmediately be able to start knocking out people and make some prisoners as a result. Also a good way to get some money early on.
Since you want to be a 2-handed crossbowman on foot (this can be considered as a 'Mettenheim-style' playthrough I guess), this settles your combat skill investment. Concentrate on Ironflesh, Power Strike, Weapon Master and Athletics. Imo you could still ride some **** horses just for the time you travel on the world map, because that increases your party speed considerably. And dismount at the start of battles of course. Having 3-6 pack horses in your inventory would also help you negate the speed penalty on the map due to the various loot and items you'll carry.
You'll probably want to eventually max Looting, Trainer, Persuasion, Leadership and Prisoner Management skills, and aim for as much party skills on your character as possible. The most important in my opinion are Surgery (soldiers rarely die) , Pathfinding (catch enemies even when your party is bigger) and Engineering (instant building siege ladders). Wound Treatment and First on your character is the second next important. However Tactics, Spotting and Tracking can be ignored, and companions can handle those. Trade skill is your call. Since you'll start with 6, it won't be hard to max.
Eventually grab yourself a good 2-hander and a crossbow. I suggest to aim for a Sapphire two-handed sword and a Mettenheim Arbalest in the end as your main weapons. You could go for a Ruby two-handed sword as well, but on foot, speed (=Sapphire) is much more important imo, as you can stay alive longer if you are the one who lands the first blow against all enemies (since you'll be dueling a lot). Also, it's much easier to just spam attacks, without the need to block for the most part. You'll be able to one-shot most common enemies with a Sapphire 2-hander too, in which case, the Ruby version would be an overkill.
Considering stats, if you don't mind to start the game with a little grinding and choosing your opponents very carefully, then you can aim to max INT for the maximum amount of skill points possible. If you'll only put stat points in the other 3 stats until you reach 11 STR, 11 AGI and 8 CHA, and then put every other stat into INT, then you can reach the 63 INT cap by lvl 47 (with achievements and the corresponding book included). With all the achievements taken into account, your charisma will match your strength and agility eventually. For increasing STR, AGI and CHA, always drink Elixirs of Arkon for Qualis Gems. But I advise you to delay those for as late as possible whenever you can get on with given stats. Simply because it is easy to get weapon profs early on, and later, when it will be harder and harder, then elixirs can come for gaining profs efficiently in the long run.