I thing you will struggle to get a single cheap turgo that will work well with that head and flow, and the low head turbines tend to be more ££ (peltons are better with more head).
I had never considered a Turgo due to the low head - instead was looking at something similar to this youtube video => V82SVeVXKcA
I noticed while reading through the Powerspout documentation that they have 2 new versions on their way
1. In Pipe turbine
2. Low Head Mini
I think the in pipe turbine is similar to the one in the video
This is from the Powerspout website:-
3.3. "In Pipe" turbine coming soon
Powerspout have developed and are testing an "in
pipe" version of the LH turbine that uses "positive
head" from a feeder pipe in addition to the "negative
head" of the draft pipe.
The main features are:
Same top end as LH and TRG products
Common propeller & driveshaft
Positive head - bolt to PVC flange
Negative head - glue to the PVC expansion
adaptor
Debris cutter included above blade
150mm inspection/cleaning opening
Maximum power 1.6kW
Maximum positive head = 6m (minimum 1m)
Maximum negative head = 4m (minimum 1m)
Flow range = 20-50 lps
Automatic grease canister included
Ceramic face seal and ceramic bearings (production version only)
Efficiency data – awaiting R&D test results
As these turbines can operate on +ive and –ive head, it will operate at up to 10m total head
and up to 1.6kW each.
3.4. LH-mini coming soon
PowerSpout have developed and are testing a smaller LH turbine that
uses a 100mm propeller instead of the 154mm used on LH turbine.
We expect this will operate in the 10-20 l/s flow range. The LH was
designed for the 25-50 l/s flow range.