A full computer for £35! Is that not too good to be true?
It is true, but also probably not quite what one was expecting. One should budget to spend at least twice £35.
Currently £35 buys a 2GB Pi 4 board. 2GB is certainly sufficient, although do be aware that one cannot upgrade later to 4GB. If one wants 4GB, one must pay £55 initially, but arguably this is not necessary. Earlier Pis did not have 4GB. From late May 2020 there has also been a 8GB option, for around £75.
However, there is a lot missing before one can use a £35 Pi.
It comes with no storage / operating system at all. The OS, Raspbian, may be free, but expect to spend £6 on an SD card with Noobs pre-installed. (16GB, UHS Class I, C10, is adequate.)
It comes with no PSU. Unless you already have a USB-C PSU capable of delivering at least 3A at 5V (or 5.1V), you will need to spend about £8 on the official one. (Note that an 18W USB-C PSU might not be sufficient, for some will supply either 2A at 5V or 1.5A at 12V. The Pi 4 does not support the higher voltages (12V and 20V) allowed by the USB-C specification. The voltage negotation part of the USB-C protocol should prevent damage to the Pi.)
It comes with no keyboard or mouse. The official one, available in white or black, is about £22, and it is not brilliant (the keyboard is not adjustable for angle, and has very short key travel). Any standard USB PC keyboard and mouse should work though, but a pair generally cost at least £20.
It is not unreasonable that it comes with no screen, especially as it can be plugged into any TV with HDMI input. It would like a Full HD TV (1080p), but should be usable with a "HD ready" model, i.e. 1366x768. (It can also support the resolution of a 4K TV, though I am not in a position to verify this.) Instead of a full-sized HDMI socket, it has a micro-HDMI socket. Expect to pay around £5 for a micro-HDMI to HDMI cable.
The Pi will include audio with the HDMI signal, and HDMI is its default audio output device. A TV will reproduce this, but some HDMI PC monitors lack speakers. The Pi has no speaker of its own, and its 3.5mm analogue audio output is not of the best quality.
Finally it is useful to be able to read and write the SD cards on something else, if only so one can re-install from scratch by downloading a new copy of Noobs. So a micro SD to USB adapter could be useful. (The Pi has only a single micro SD slot, so without this even the Pi cannot read or write to a spare SD card, but only from and to the card from which it booted.)
Cases are not needed, and many reduce the amount of cooling available to the Pi. A bare Pi is a little fragile, so do be careful with it.
So after arranging use of a spare TV, and perhaps spending closer to £75 than £35, have we got a usable computer for our money?
Not really, but this is not entirely the Pi's fault. If by "computer" one means something on which one can read emails and browse the web, then there is still a network connection missing. And the Pi, by default, loves to update its software packages at every opportunity, which leads to large downloads. Of course you might also wish to install extra software, as there is a lot of good-quality, free, software for the Pi, not all of which comes pre-loaded. This will need a better internet connection than a 4G link with a low data cap. Standard home broadband is absolutely fine, but I know of no way of getting that for even just a year for £75.
In theory one could take one's Pi to a friend / school / library(?) every time one wished to download something, and still do useful work on it when it is disconnected from the internet. This would however be regarded as an unusual way of working.
Models prior to the Pi 4 have full-sized HDMI sockets. It is more likely that one has a spare full-sized HDMI lead, so this might save a couple of pounds. They also use micro-USB for power, rather than USB-C,and it is more likely that one has a spare micro-USB charger from an old mobile phone. The recommended 2.5A (for the Pi 3B+) is more than most phone chargers can supply, but one may get away with less.
The basic board for the Pi 3 B+ is slower than the Pi 4 B, and has only half the memory (1GB), but tends to cost the same price. So the saving on HDMI lead and PSU needs to be significant before it looks attractive.
The Pi 3 B might be a couple of pounds cheaper than the 3 B+, and has slightly slower CPU and ethernet (both wired and wireless).
The Pi 3 A+ is certainly cheaper, but it has no wired ethernet port (wireless only), half the memory again (512MB), and only one USB port, rather than four. Fortunately the official keyboard and mouse includes a USB hub in the keyboard, and would leave two ports spare after plugging in both keyboard and mouse.
Be aware that the basic Pi Zero has no networking at all: not wired, not wireless. And that it offers a single micro USB port (no full sized ones), and a micro HDMI port. One might need additional adapters. The Zero&nsp;W does have wireless networking, but has the same port selection.