The SWT service from Waterloo to Poole is ok, it's not worth the upgrade to 1st though really. There's no WiFi for instance. The time you are leaving is a 10 carriage service with two 1st class coaches. These have two seats down one side and one seat down the other. It should be an at seat buffet service throughout the train, but given the time you are travelling it might be buffet car instead.
The train will leave from platform 7,9 or 11 provided there are no issues, and the 18:05 should arrive at Poole around 20:05. After Southampton, it will become a ghost train. Provided you are at Waterloo by 17:50, you'll get a seat, sometimes, this service is standing room only and will be busy especially before a bank holiday.
In slum class, sit in any of the seats which back onto the bulkheads (the main sliding doors into each carriage), these have more legroom than any of the other seats in the carriage. The 2nd and 7th carriages (from Waterloo) have one single seat in the isle which are often highly desirable in cattle class. This is because the disabled toilet is also in this coach.