Actually, that guide doesn't answer the OP's questions very well.
Like all melee DDs, SAM look for good weapons, and upgrade as often as better weapons become available. That is the highest priority. This is a pretty simplified list, but should be more or less accurate at lower levels for what to look for in gears:
1. Weapon
2. Accuracy
3. Attack
4. Strength
5. Dexterity
6. Defense
Somepage's database is the best way to search for available gears:
http://ffxi.somepage.com/itemdb/advanced.php
As for invite rate, it really depends on which other jobs are available, and how many people there are seeking in your level range. Just realize that for most levels, people want tank, healer, and support first, and you will play second fiddle to them and compete with a lot of other DDs for the remaining slots in party.