If you are 17st and mates are similar then anything designed for 2 such as the 152 or PA38 is really a non-starter!
I would have thought a PA28 would be the obvious if predictable option. Plenty out there. Seek advice in inspecting any aircraft; need to look at the time the engines/airframe have, scruitinise the maintenance, see what bills are looming. If it is genuinely a person selling their share in an established group, with a healthy engine fund then you should be fine. If it's an owner looking for a cash injection to keep a tired old bird in the air then excercise caution!
Look in the classifieds for some examples, for a typical quarter to a sixth share in a reasonable PA28 anything between £3K to £10K could be paid with a fixed monthly cost which sometimes includes an hour of flying, then pay per hour wet.