Re: Changing website hosts
There's a number of factors here, the main being:
1) Depends to some extent how the SEO has been done, i.e. If a majority of it relys on off-site SEO then as long as the changes are for the better then you should be ok. Totally redesigning the site will cause Google to relook at it differently, just ensure it is for the better not worse.
2) Depends on how well the new site is put together in that you understand what gives it it's high ranking in the first place. Does the domain name play a part in that high ranking, will that stay the same?, do the page titles, keywording, page descriptions, page text content, layout, navigation have anything to do with it's ranking high... There is a lot to think about here.
3) Depends, without insulting you as I don't know your capabilities what your background is in site construction, SEO etc....
Just some food for thought...
Don't aim for success if you want it; just do what you love and believe in, and it will come naturally.