Scepter makes Refresher completely useless since his ultimate would have a 2 second cooldown after the scepter upgrade. Scythe of Vyse and Linkens are great choices, maybe even a Skadi or a Radiance if you're overfarmed.
I've found Invoker to be ad an odd place, myself. Played him quite a bit in the days of DotA. His strength is his versatility... and his weakness is that he doesn't really have any strengths. He can nuke, but not as well as a dedicated nuker. He can carry, but not as well as a dedicated carry. He can even support, but nowhere near as well as a dedicated support.
Being able to swap spells on the fly to suit the situation takes a lot of playing with. The three main builds I've seen or used usually involve the following:
Q/W for attack speed, infinite lane sustain, and nasty Cold Snap abuse, occasionally with Radiance/Ghost Walk shenanigans. Alternately, for Tornado/EMP usage, but people tend to stick to one gimmick at a time due to early mana issues, and it takes a little more skill to pull off.
Q/E for Forge Spirits + Cold Snap, with high autoattack damage and sustain
W/E, with a point in Q to abuse high damage last hitting, Meteor+Deafening Blast to push them in the highly damaging portion for longer. Also, Sunstrike.
Lots of ways to play him otherwise, and Ghost Walk makes escaping just about any situation a piece of cake. Find a style that works for you and learn it, and you'll have a hero that can do just about anything.