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Not a full guide but just so people know, you can upgrade a weapon and it counts.
Also, you could save up, make a save and revert back to it if you didn't want to stick with 1 item fully upgraded.
Use your Eagle to find titanium in the areas 90 power and above, like East Anglia, and Oxfordshire, and buy from shops titanium and iron when its available...you usually have enough leather.
The last couple levels for me was 14 and 18 titanium fir a bow so needed 32 for last 2 levels. (Will be more for other types of weapons)