Santi,
I don't now for the legal aspect if Avast allow home edition used by a PC repair company. Because here is what avast told us:
"avast! Home Edition - Free Antivirus represents the best FREE antivirus protection currently available on the market. This edition is FREE OF CHARGE for non-commercial & home use only. Both of these conditions should be met! If you are not a home user or if you use your computer for business purposes, we offer a wide range of more suitable products providing higher data security, starting with avast! Professional Edition."
I'm afraid if your are using it for business, Avast won't allow the use of Home Edition. If you are prefer to use Professional Edition, your company could purchase a lot of license at once. Your company maybe interesting become avast reseller for the better way.
But if you preinstalled avast on your clients' computers, maybe just let the Home Edition without a license key, and let your costumer to get the license by their own way.
I thought only for personal use, because its okay to use single license key for all PCs we have, no matter how many PCs are there. So there is no way a personal user of Home Edition can get multiple license while one can use a single license for all his machines.
If you are in doubt, just contact support@avast.com for clear information.