You are going to need a fully managed VPS provider then. Unfortunately, many providers have a different interpretation of the words "fully managed" and thus offer different things under their fully managed plans. I would recommend that you look for a few different fully managed VPS providers within your budget and contact their sales department to ask if they will help you install the script
I'm doubtful you're going to find anyone who offers this as an upfront service - especially considering that, according to that site, there are only 300 people running in that script in the world.
You might consider finding a good host and then paying someone to install it for you (i.e., contract with a freelance sysadmin), or perhaps contact the provider and ask if you can buy some sysadmin time to install that.
xfilesharing is easy for installation and there is a very few installtion requirements and ALL there requirements are same features in any cPanel server / VPS .. .. but you will need to find a stable host and get a good deal of the bandwidth / storage ..
check the VPS offer section in here .. and try to contact the companies that you find they meet your BW / Storage requirements .. and sure you will find some companies that could install it for you ( but will be hard to find any company to keep managing it for you )