I see no reason why the 3tgteu gear can't be fitted onto an 18rg. The air flow meter diameter is identical. The 18rg system is the same system as the 3tgteu (Ljetronic/nippondenso) the advantage the 3tgteu has over an 18rg ecu is the boost enrichment good if you ever wanted to turbo charge the 18rg but can be left disabled. Another option for you ben is the 2s injection gear off the camry leaded version, it runs on a 2 litre and is almost identical to the 18rg stuff, and would work just as well, most wreckers have them. The 18rg manifold is a little more difficult to find.
just another note, all the sensors remain the same between the 2tgeu, 18rgeu, 3tgteu, 2se (1984-5 leaded camry) as they run variants of l_jetronic you can even interchange parts with bosch systems.