Hi, to me portrait and street feels a bit contradictory with same focal length... thus you might want a zoom going from wide to short tele such as 16/17-50. The Tamron 17-50 is very sharp and fits in your budget. The Pentax 16-50 is expensive and has the SDM burden. The Sigma 17-50 is not as regarded.
For the Tamron, avoid the VC version (rumored to have a modified, lesser optical formula), prefer a made in Japan one, and check it for focusing issues before buying.
See samples of what it can do
here.
Alternatively you might go with two manual lenses such as M3.5/28 or K3.5/28 for street, and any Pentax 50mm for portrait. You'll get better rendering, Pentax colors, and superb bokeh at the cost of manual focusing. Budget would be much less than 300$ as well.