...yeah, so whats the problem?
TBH all you need is to get the three(workshop, office & laptop)on the same network
I assume the internet connection is fed to the house or home office?, is it via wireless router situated in the house, or is it direct lan ethernet connection to the desktop?
Getting the office and shop networked is a piece of cake, you have three options, 1. you hard wire, 2. you situate the wireless router in such a place in the house as to bring the signal in range of the shop either by physically moving it closer, or by purchasing add-on antenna. 3. get a cheap usb network card and one or two long usb extension cables to bring the network card as close to the shop as possible, preferably line of sight.(ad-hoc connection, [internets>desktop]-----[desktop>shop](two seperate networks)
For on the road a mobile broadband subscription for the laptop?...remote desktop to office and shop now possible and depending on your ability it may be possible(someone please clarify???)to map the drive peechtree uses over the mobile broadband network, having one single working folder for peachtree serving all pc's.