There are soo many choices out there but the Merkur 34C is a good razor to start with and it seems to work with most blades. It would be a good match with the Astra SP.
You may have to adjust the lather but that will come as you progress. The Shave Wiki is a good reference for technique.
The Astra SP is my favourite blade, it's very popular because of it's smoothness. I have no problem with the recommendation of the Merkur 34C, in fact I have the 34G version in gold. But if you want reasonable quality for a little less money, the Parker range are good. I have four of their razors and not had any quality issues. Even less expensive is the Weishi TTO. They do chrome, gold and black versions. You won't find too many fans on here for the mild Weishi, and I appreciate that it's too mild for some, but I really like mine and if you decided to invest in a more expensive razor later, you could keep the Weishi as a travel razor. At those prices if you lost it or damaged it, it wouldn't ruin your day. If you think you need an aggressive razor, forget the Weishi.
What is your skin and hair like? e.g sensitive/thick growth etc?
As you are already using a straight/shavette, I will assume you are happy with blade exposure and like a close shave. The Astra SP in my humble opinion, is a reasonably versatile do it blade.
I would say you probably want to be looking at a moderately aggressive-aggressive razor. (I also use a feather artist club SS when I am not rushed for time).
Muhle R41 - open comb known to be one of the most aggressive or as I like to call it, highly efficient. Got to have something like this on the collection. IKON Tech Shavecraft (aggressive models ) if you can find them. Above The Tie H1 (closed comb but comfy and efficient) Muhle Rocca
The other recommendations like Razor Rock with adjustable base plates are all round good value.
A good starting razor, forgiving and capable of a three pass BBS, for me anyway. The Edwin Jagger DE89 or Muhle R89, same head. Used with a neutral shave angle, guard and top cap both in contact with skin thru the stroke gives me a BBS irritation free shave using a Derby, Astra SP or Feather. If you want something more aggressive I agree with the razors DSK listed above. Good luck.
Double edge razors can be mild or aggressive. With a mild razor, the sweet spot can be harder to find, but the lower blade exposure makes it a little better for newcomers who are moving on from cartridges, where they are used to applying pressure. A more aggressive razor has a wider angle tolerance, but can be more "bitey" for the inexperienced. As you're coming from shavette, and are unlikely to be heavy handed like someone coming from cartridges, I'd suggest looking at a more aggressive razor. I'll let others guide you as to which may be more suitable, as I have limited experience over different razor types out there.
As for blades, if you already have Astra, start with those. You already know how these should feel on your face, so you know what to try and emulate in the new format. Once you're confident in your razor handling, then move onto exploring blades.
Thank you all for your suggestions; this is why I have really enjoy B & B! Being as impatient as I am, I have ordered the 34C. After being on the forums for a bit, I have a feeling I will now be adding DEs to a future collection.
If want to get a good feel for DE shaving from mild to aggressive without buying lots of razors go with an adjustable. My opinion the best entry level adjustable when first starting is a vintage Gillette Slim. It has everything you would want when starting out and can be had for under $20. Once you find what setting(s), blade you prefer then from there you can figure out what non adjustables to branch out into based upon that
There are also though that would recommend using a non adjustable until you learn how to shave correctly with a DE razor instead of constantly changing settings but in your case you already know how to since you already use a Shavette. You should already have a lite touch and be able to shave with little to no pressure so it is just matter of making adjustments for the angle of attack based upon how aggressive or mild the DE is for each type of pass like WTG, XTG, etc.
I think you made a pretty solid choice. I started with a Weishi just in case I wasn't a fan of DE's and then moved to a 34c. I wish I just went with the 34c in the first place but now I have a decent travel razor which is exactly what Joe Namath suggested. Gillettes of auctions sometimes have to many variables unless you buy it from a B/S/T group.
Just out of curiosity, what did you think about using a Shavette?