welcome to the club chris, yes u can pick up discos quite cheap now just check the norm door sills, boot floor, and wheel arches they tend to rust. with £800 spare i would buy all terrain tyres about £300 spent or mud terrain if serious offroading, diff guards £20 - £40 a pair, sump/steering guard £60, if you wanna wad through deep water get a snorkel £60-£120 dependeing on if u want mushroom or safari, spot lights £50 for a decent pair, cb radio £80. if you get more money look into lift kit complete (includes braided brake hoses, radius arm etc) winch bumper and winch its an endless possibility
hope this helps jay