I've built a few frames in 3AL-2.5 (Grade 9).
1. Cleanliness is paramount - Make absolutely sure your grinding & cutting tools are new or haven't been used with other metals. Even fingerprints on joining areas of the tubing will contaminate the joint during welding.
2. Ti is brutal on drill bits and cutters such as hole saws. They need to be quality and sharp.
3. Successfully bending thinwall won't be easy without access to a very expensive mandrel bender. I suggest you try to eliminate bends wherever possible.
4. Ti burns in the presents of oxygen. I know,... in the process of turning a piece on a lathe, I was creating a sizable pile of curly Q's. Long-story-short, the strings eventually caught fire and started burning under the lathe bed - real pretty colored flames. Of course, this means that back-purging is manditory.
5. Be clean & careful.