Hi Audiopilot,
I got some time to kill in the train so here goes my rembling:
I would advise you to try out the InEar thing for more than just one gig. It took me like 10 gigs to get used to them.
I also would say that you should have a full mix and not just vocals as a signals for your InEar.
Not a full mix but I (play in a 8 piece band have the following in order of loudness on my InEars)
My Voice
My Guitar
Kick and Snare
HiHat
Clicktrack
Leadvocals (most of the time not me)
Keys
Other vocals
Bass
Overheads and Toms
Backing Tracks
And almost no Trombone, Trumpet and Sax
The mix I hear is something between the perfect mix to perform for me and something that I also enjoy.
If I don't have enough time for the soundcheck I just get drums essentials, click, my voice, guitar and lead vocals.
I would still advise you to leave a cab or a monitor on stage for your guitar sound.
Natural feedback is a big part of most guitar sounds and I myself hate the loss of sustain if I solo without a cab.
Ive been doing gigs with in ears for about 8 years and in the long run it was worth it for me.
I still use my Shure SE315 and a cheap transmitter by LD Systems. And I don't think that I'm missing out that much. About 500 bucks together.
I play gigs in front of 30-3000 people.
I don't have a custom mold because the normal one fits me very well and I'd rather buy more guitars than invest money into something that is working just fine.
Whatever you decide to buy have fun and your ears will thank you. Especially if your drummer hates his cymbals as much as a few drummers I had the joy of working with.