Most (if not all in my experience - but that is in Europe) 50Hz TVs also have 60Hz compatibility - it is just the reverse situation which isn't the case (i.e. Most sets sold in 50Hz territories also handle 60Hz, but many sets sold in 60Hz territories DON'T handle 50Hz stuff).
There is also the difference between whether a set has a tuner capable of receiving OTA analogue in the various standards (PAL B/G, PAL I, SECAM L, NTSC M etc.) and whether it can take an externally tuned (say via a VCR) composite input (or component) which will be just NTSC, PAL or SECAM - or a 50 or 60Hz component/HDMI signal (with no composite encoding)
In Europe all HDTVs with the "HD Ready" logo have to support both 50Hz and 60Hz 720p and 1080i HD inputs - and I haven't come across any that don't also support 480i/p at 60Hz as well as 576i/p at 50Hz. Even every European SD "PAL" TV I've owned since the early 90s has has both an NTSC and a PAL decoder in it - and syncs to both 50Hz and 60Hz SD inputs from NTSC and PAL composite sources as well as RGB (European SCART connectors cope with SD RGB outputs from digital set top boxes and DVD players - avoiding the quality drop that composite and S-video introduce).
Since I went HD all of my HD sets (all licensed as "HD Ready") bought in Europe have coped with NTSC and PAL composite, and 50 and 60Hz HD inputs (all of them were also 1080p at 50 and 60Hz - and the most recent one also 24Hz) and have had multistandard tuners to cope with PAL I, PAL B/G (and SD MPEG 2 DVB-T - the pan-European SD digital TV standard)
Similarly almost all DVD players sold in Europe will output both 50 and 60Hz natively. (Meaning 50Hz "PAL" DVDs replay at 50Hz and 60Hz "NTSC" DVDs replay at 60Hz - as the TVs in Europe will accept both natively)
Standards conversion at the consumer level is usually pretty awful - particularly on large screens. Best avoided unless you don't care about picture quality.
According to one list I've found online Angola uses PAL-I for OTA broadcasts - which is actually one of the less common variants. We use it in the UK on UHF only, and the Republic of Ireland uses it for both UHF and VHF - and the main other locations that use it are either current/former British Dependent Territories or Commonwealth countries.
I'd look at a European "HD Ready" TV - and if Angola is PAL I I suspect these models will be the ones sold in Angola anyway?
These days many TVs sold across Europe are fine with all of the main European analogue standards : PAL I, PAL B/G and PAL D/K (the systems differ in sound carrier frequency and vision bandwith mainly) as well as many having SECAM L (French system)