What is the most trouble that British tourists are causing? What was the purpose of saying " Many from the UK" if clearly and obviously they are not the only ones?
Malaysia is the most difficult embassy to gain approval for the DTV e.g 3-6 months of 500K, 6 months of accommodation booked in Thailand, Jakarta i hear is more friendly to Chinese applications recently. Tiger Eye in Bangkok, and Muay Thai Fever chiang Mai are popular MT gyms and DTV compliant.
There is no minimum income requirement, Malaysia required 6 months of 500k THB. Vietnam approves with a final balance of 500K THB and 3-6 months statements. Check the main DTV group on FB, loads of Vietnam application tips there.
Buying a condo does not give you permanent right of stay in Thailand. You can buy a condo with a tourist visa or visa exemption. Permanent residency requires you stay in Thailand for 3 years nonstop on the same visa extension before application. You should look at getting a retirement, business, employment visa..