A lot of us use old PDAs as they often have better screens for use in bright daylight - mine cost me £30 on ebay and the software I use is free.
I have brought two expensive hi-tech things, an EW Microrecorder and later an IGC approved Flarm (so traces from it are valid for badge claims). I use the EW if I fly club gliders as I like to have a trace for each flight - and if the times on the log sheet are wrong I have some evidence to back me up as I tackle the man that puts the details in. (of course I should check them myself)
The only other glider-related things beyond insurance and annuals I've spent money on were new tyres and inner tubes, which are hardly hi-tech, and a small amount on some wiring to get 12v to the Flarm and EW, and the NMEA from the EW to the PDA - that cost me a good bottle of wine to the chap who wired the tiny tiny plug for the PDA.
The glider came with the LX4000 which in it's day was the bees' knees of flight computers.
However some people have brought a ClearNav - ££££! It's an excellent device though, the screen is genuinely readable in sunlight, and it's easy to upload airspace and turnpoints using USB - a lot of older devices are serial only which can be a problem with modern laptops that no longer have serial ports. Some GPs have had lots of problems with USB-Serial adapters that aren't fully-featured so don't work for uploading & downloading PDAs and so on.
I've seen the NATs device and didn't think it would be very easy to read in the cockpit, plus
AFAIK we can't put the TPs in.
The other up-and-coming device seems to be the Oudie but I've never seen one in person.
Relief - think plastic bags or pee tubes... And it's essential, as you need to drink if flying for 5 hours.
I had no idea they had fly-ins at Lasham - I know they do at Popham, but getting a DC3 in and out there would be a bit ambitious. Saw the Coventry-based DC3 at HusBos last year, take-off and landing runs (empty) are very impressive. It attraced a lot of attention.