1. You can spend marks to get different sets of armor (I collect gloves with reload speed perks for the different primary weapon types, for example) so you can mix and match You may also get into a situation where you need ascendant shards or energy. Buy a tower piece of gear and break it down. You could also buy planetary materials (spinmetal, helium, spirit bloom, or relic iron) They also sell sparrows, that can be further upgrade with an item from Xur. Legendary engrams are pretty rare. I'd say the rate they actually turn into gear has been around half the time for me, since they did the engram update a while ago. 2. Most of my purple gear with different mods comes from faction reputation packages and nightfall strikes, and the occasional random drop. You can get purple engrams from weekly heroic strike, and rarely from the daily heroic story. Only iron banner legendary guns could get their mods 're-rolled'. If you see a tower gun, but don't like the mods, the only way of getting different mods is a random drop. If you get gear in a reputation package, it's gear from that factions stock.