I've edited the title for you (@9 to 29).
3 WPG is ideal for most plants should give you good growth IF you get all the other aspects of the set-up correct. Ensure you use good quality tubes with reflectors. Aim for full-spectrum, high output. These are initially more expensive but a worthwhile investment if you want the best chance of long-term success.
Your questions -
1) Personally I would go for an external canister filter. These provide more biological media, are easier to maintain than internals and are visibly less intrusive. Go for a model which is rated above your tank volume i.e. Fluval 204. Eheim are a good make too. Ensure your filter output is positioned so that it doesn't agitate the surface, this minimises CO2 loss.
2) Good substrates - Fine gravel mixed with laterite (cheaper), Flourite and Eco-Complete (more expensive). I have no experience of your current substrate so cannot comment.
3) All the commonly available liquid ferts are fine for most situations. Regular water changes combined with fish food and waste will provide most nutrients. If you stock very lightly and/or experience very good growth then you may need to add macronutreints (N P K). I wouldn't worry about this just yet.
4) YOU NEED CO2 with 3 WPG otherwise you will have algae issues very quickly. Get your CO2 set-up and running BEFORE planting and lighting. Aim for 20 to 30ppm constant. DIY CO2 is fine. I recently hepled a few people out with my own design and recipes. Here's the info.
You need -
2 litre plastic fizzy drink bottle with cap
Silicon sealant
6mm drill or similar
3 metre silicon airline/tubing
lots of sugar
dried yeast
bi-carbonate of soda
teaspoon
funnel
weighing scales
measuring jug
water
1. Drill 6mm or slightly less in bottle cap.
2. Insert airline an inch or so in hole and seal with sealant.
3. Allow to dry.
4. Recipe - 400 grams sugar, 2 level teaspoons yeast, 4 level teaspoon bi-carb, 1.6 litre tepid water.
Pour in sugar, add yeast, add bi-carb, add tepid water. Use funnel if necessary (less mess).
5. Attach airline to your filter inlet or diffuser.
6. Watch CO2 bubble away after a few minutes.
This mixture should give a fairly constant output for approx 7 to 10 days. Longer if using less yeast.
Bear in mind you will need LESS yeast (you will overdose CO2 if you don't). Keep all other quantities the same. I'd suggest reducing the stated yeast dose to 1/2 level teaspoon to start with. Experiment with different levels to acheive required CO2 level. You need to buy a good pH and KH kit to test for CO2. There are many CO2/pH/KH tables available on the net. Ensure your KH is at least 50ppm (3 degrees) before dosing CO2 otherwise pH crashes will be possible.
Finally and very importantly you will need to plant heavily with fast-growers from the start. This will ensure your system balances rapidly thus preventing algae from getting a hold. Cover the substrate with at least 50% of plants. Ideal fast-growers; Hygrophila species, Ludwigia species, Ambulia, Water Sprite, Egeria species, Tropical Hornwort and floating plants. Once your tank has balanced and you see good growth with little or no algae then you can gradually replace the fast-growers with slower growing, more delicate/demanding species as per your taste.
Hope this helps and good luck.
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