Because I'm mainly work out in the field, I use a laptop, and for me, the lighter the better. I have a Microsoft Surface Pro 2 which offers a good balance of speed and weight. But there a too many to choose from. Ultrabooks are great too.
A small screen may be too small for some, but you will have to balance that against the weight of the laptop if you are carrying it between appointments. If you’re home based, it will be less of a problem as you can choose a larger screen.
For printing out in the field, HP have made brilliant 'mobile' printers for years. The first incarnation I bought was the Officejet 100 Mobile printer. The predecessor was the HP470WBT and before that the HP460WBT. eBay has them periodically, usually under used.
As long as it has Bluetooth printing capabilities you won’t be hunting for a printer cable every time you need to print out a receipt.
They are a little pricey but they should last years and I only use one ink cartridge a year!
But any printer will do if you are home based.
For a backup, why not consider Fineprint? Fineprint allows you to reprint to another printer. Handy if your usual printer has just bit the dust and you need to reprint missed invoices and receipts.