You can get a rough idea by looking at the shares advertised for similar aircraft and multiplying the monthly rate by the number of members. Groups do sometimes have to ask their members for cash to cover the unexpected, particularly when the annual inspection comes round.
£16,000 is the bottom of the market even for a C152 so the next annual inspection will almost certainly cost a few thousand.
The other thing to look at is the life left in the engine. If it's done less than 2000 hours budget for an overhaul at 2000 hours. If it's already done more, it may be OK for a while yet, but you don't know. So have enough in your back pocket to replace/overhaul.
If by 'viable' you mean comparable in cost with renting or sharing, yes, only if you do about 5 (some say 10) hours a week would owning be viable. People buy because the convenience of being able to fly when they want and being able to take the aircraft away for days at a time justifies the cost.