I'm so much excited as scared now

Could you please help me a little and answer few silly questions ?
1. Please check my design.
There's less space on the left side due to door rail. can logo be smaller here and bigger on right side of the van ? or would you rather make them both same size
any other advice on my design ?
I tend to make designs the same size but obviously it will depend of available space and content. Don#t be afraid design something that goes over joints, just slice the vinyl and tuck the edges round - use a little heat if the edges seem reluctant to stick. From a distance the eye will pretty much ignore the join but it will notice something off centre or letters with bad spacing.
2. As van's panels are not square, should I position designs horizontally to bottom lines ? or...
Modern vans don't seem to have parallel lines any more (new Transit being a prime example) but there generally is a dominant eyeline, it may be a trim or it may just be a case of making it look right with the line of lettering and then working from that. Often a good start point is to measure the height of either end of the main panel and then split it so with the Berlingo measure one end of the panel and mark the middle, do the same the other end and join the two, take all your measurements from that line. At the end of the day if it looks right it is right:)
3. Can vinyl be applied on a rainy day ??
No, if the backing paper gets damp it will de-laminate and you will have a job getting that off the adhesive of the sticker
4. Can vinyls be layered ? (that's for the future)
Yes but make sure you use the same grade as a monumeric will shrink more than Polymeric which will shrink more than cast
5. Any advice where to practice?? I was thinking about bathroom wall (gloss tiles), but not sure if it's the best idea

You an practice on any smooth surface
Can't make my mind if I should stick vinyl wet or dry.
It is worth practicing both techniques but I tend to use wet if very hot weather or large sticker and dry if cold or small sticker or logo with fine detail
Also I'm gonna do this far away from home so there will be no repeats :/ maybe worth to cut everything double just in case ?
Not very profitable if you are doing two of everything twice:) so be brave and take your time, once you have a few under your belt you won't worry about it and I bet you'll make less mistakes
So excited so excited

If you are excited about putting stickers on you need to get out more:)
Have fun, enjoy and take pride in your work