This depends on your preference and the layout of your home. Vented tumble dryers are generally cheaper to buy but need venting outside. Condenser tumble dryers can be placed anywhere in your home and collect water in a tank, which you must empty.
The main difference is how they treat the extracted water. A vented tumble dryer uses a hose to channel this moisture outside. While a condenser dryer will collect water in a reservoir for you to empty after each cycle.
Heat pump tumble dryers are by far the most energy efficient tumble dryers. They have the highest energy rating and use less than 50% of the energy required by a conventional dryer.
As a rule, a 7kg tumble dryer is suitable for an individual or couple. It will wash approximately 35 T-shirts in a single load.
Yes, proper ventilation is vital. Where possible, place your tumble dryer near a window or patio doors to allow cool air to circulate.