I don't think you need a barlow, though others will disagree. I tried
an Orion Shorty Plus, supposedly an acceptable quality item, and when I
had combinations of eyepieces similar powers with vs. without the
barlow (eg a 25 mm plus barlow vs. a 12 mm), I slightly prefered the
image without the barlow in each case. Admittedly, the difference
wasn't dramatic.

In any case, a 4.5" newt isn't going to give great images above 150x.
That "675x" in the advertisement is COMPLETELY DELUSIONAL!!!! A
commonly stated max power is 50x per inch, but 25-30x per inch is
probably more realistic. IMHO, it's always more enjoyable to be a
little underpowered and have a clean, steady image than to be a bit
overpowered and be constantly distracted by bad seeing, tube currents,
and diffraction effects.

So, you don't need an effective focal length under 6 or 7 mm.
Unfortunately, the 6 mm wide-angle cheapy eps (sold under various
names, including Antares as mentioned by JC and Orion Expanse) have a
bad internal reflection, or at least they did when I bought mine. That
makes it almost unusable on the moon, but otherwise it's nice, with
bigger field of view and much better eye relief than the ortho or
plossl. Others in that series are supposed to be better in terms of
reflections, see reviews on Cloudy Nights or search this group.

So, I might suggest a 32 mm plossl, 15 mm Antares/Expanse and a 7 mm
ortho or plossl. Or if you really want to fuss with a barlow, a 32, a
20 mm Expanse, a 12.5 mm ortho (I have a UO and like it a lot, despite
the small field) or 12 mm plossl and a 2x barlow. If that fits your
budget, it does give a lot of options. You could omit the 20 mm and
not lose much.