If I am right in saying, the most likely manufacturer for any new trains for Southeastern has always been Hitachi.
That was only due to the requirement for bidders to partner with a manufacturer (or plural) for rolling stick and signalling (inc TMS) particularly for the later due to DfT desire to experiment with PFI style sub-contracts for those elements.
The only sensible HS1 rolling stock option is for another batch from Hitachi (7-9 units).
The Hitachi cost for metro type units is higher per vehicle than Bombardier or Siemens (Bombardier being cheaper than Siemens in overall cost which is why they won Anglia, SWR and LNR). Bombardier losing Thameslink forced them to have a rethink about Aventra mk1 and have the design refresh leading to the "mk2" design actually built.
The Desiro City design was targeted at Thameslink bid requirements and things have moved on since then (much of the DfT thinking for that was done 12-13 years ago).
Hence like SWR the cheapest overall option may be to go for a uniform fleet of Aventras unless Siemens go for a refresh of Desiro City at which point they have less in common with the 707s.
The much previously discussed lack of hand holds in the vestibules (which gets them bad passenger standing density points scoring) and has no easy fix effectively excludes them from SE unless DfT relax their stance (unlikely).
SE are getting the 707s because:
a) they are available soon
b) Angel have got realistic on the lease costs
c) while not perfect* for metro use they are by far the best short term option to add capacity and improve dwell time performance.
*e.g. hand rail and seat spacing issues
