I've never owned a laptop so this is a naive question. Is there any problem with using a laptop as a desktop replacement? That is, is it normal and standard for a laptop to support using an external keyboard, mouse, and monitor, never needing to use the built in keyboard and display?

Sure. Many people in corporate America do just that. Laptops are standard corporate issue these days instead of desktops. This allows them to work at home in addition to at the company office, and also take their computer when they travel on company business or even into a meeting room (wifi is usually available these days). But if you need a computer for personal use and don't need to use it in more than one location, or travel at least occasionally and need to take it with you, I am not sure it makes a lot of sense.

Generally you will sacrifice some speed and/or pay more for a laptop. Every laptop I have worked on has a fan, and although it relatively quiet, it is not silent, and the fans do go bad (and sometimes make really obnoxious noises). Obviously the fans can be replaced, but not as easily as in a regular computer.

If you do move around a lot and need to take your laptop, you will need a carrying case and may want a docking station. Make sure you factor those into your budget.

let me ask you the following question:

will you ever need to use it somewhere other than your desk area?

if the answer is "no" or "barely ever" i would strongly recommend against getting a laptop for desktop use. here's why:

expensive
underpowered
smaller hard drive space for the money
smaller screen
you're paying for stuff you won't use (built in keyboard and mouse on the laptop)
they heat up too hot usually / hard to clean out
did I mention expensive?

these are just a few of the reasons I hate laptops for extended desktop use. if you don't need to bring it places, do yourself a favor and pick up desktop and save tons of green


EDIT: if you already HAVE the laptop..then in that case I would say, sure. Buy the peripherals like a keyboard/mouse/monitor and make it easier to use in doing so. That said, I would never BUY a laptop for this intended use..

Let me explain my interest. I'm comparing the options:
(a) cheap fanless desktop
(b) cheap fanless laptop

The cheap fanless desktops have similar limitations to laptops, and are not a lot cheaper. So I figure, why not get a laptop, even if 99% of the time it will be used in desktop mode?

A cheap fanless desktop can be quite powerful, and even more so if you add a single quiet fan. Look at my signature for example. A cheap fanless laptop will be not be very powerful at all.