My suggestion is that your should scrap your requirement that a lens
in the 14mm to 18mm range of focal lengths "must be of comparable
optical quality to Nikons 50mm 1,8".  Why?  Because that requirement
simply cannot be fulfilled by any lens in that focal length range.

The 50mm f/1.8 Nikkor AIS, AF, AF-N and AF-D lenses (all of which are
optically almost identical) offer outstanding resolution and contrast,
extremely low distortion and excellent colour rendition.  The only
possible criticism of these lenses is that the bokeh is harsh.  In the
14mm to 18mm range you are going to have to accept optical compromises
that are not forced on designers of 50mm lenses, and no lens in that
range of focal lengths will perform remotely as well.  

You might like to consider the 14mm f/2.8D AF Nikkor:
http://www.nikonimaging.com/global/products/lens/af/wideangle/af_14mmf_28d/index.htm
or the 18mm f/2.8D AF Nikkor:
http://www.nikonimaging.com/global/products/lens/af/wideangle/af_18mmf_28d/index.htm
Of these two, the 18mm is optically superior, but that isn't much help
if you really need the 14mm focal length - there is a *huge*
difference in the field of view between these focal lengths.

Of the independent manufacturers, Sigma and Tamron make 14mm f/2.8
lenses.  These two have performances that compare reasonably well with
the Nikon 14mm f/2.8 but both are cheaper.  The Sigma is considerably
cheaper and probably offers the best value, subject to the usual Sigma
concerns about build quality and compatibility with future DSLRs.

In manual focus lenses, Tamron and Tokina make good 17mm lenses, but
they won't offer metering with your D70.  Neither will the 15mm and
18mm f/3.5 AIS Nikkors.

Finally, why not consider a zoom lens?  The Sigma 12-24mm and 15-30mm
both cover 35mm full frame and are optically surprisingly good.  

The 12-24mm f/4 G AF-S DX Nikkor and the Tokina 12-24mm f/4 AT-X Pro
DX both cover only the APS-C sized digital sensor but the Tokina costs
less than half the price of the Nikkor.   There is also a Sigma
10-20mm F4-5.6 EX DC for the smaller sensor.  

Given that the performance of the Nikon, Tamron and Sigma 14mm f/2.8
fixed focal length lenses is not stellar, these zoom lenses are well
worth considering.  A Google search will reveal a wealth of reviews
and test data of variable quality.

For the best overall optical performance, the 18mm f/2.8D AF Nikkor is
probably the best of the lenses I have mentioned.  Good luck with your
research and I hope you enjoy choosing!